The Sixth Dialogue.

Exposition of the Newtonian universal Principle of Attraction, and Application of this Principle to Optics.


THE next Morning the Marchioness no less impatient for Attraction than she had been for all the rest, after a few short Compliments, began, It is now Time to mount our Hippogryph, and give him the Reins. He will not grow weary, answered I, for a little Journey, if I can remember certain horrible Numbers.

All the Planets revolve at various Distances round the Sun, who is almost nine hundred thousand Times bigger than the Earth, and forms the Centre of their Motion, while he remains in <149> the Tranquility of a Majestic Repose. Nearest the Sun, at a Distance however of thirty two Millions of English Miles, is the little World of Mercury, then follows the lucid Orb of Venus fifty nine Millions distant, then our Earth eighty one, Mars at one hundred and twenty three, the enormous Bulk of Jupiter four hundred and twenty four, and the vast and slow-moving Saturn, seven hundred and seventy seven Millions of Miles.

These all preserve this natural Order in their Motions, that those nearest the Sun compleat their Revolution or Orbit in a shorter Time, and the most distant in a longer: Thus Mercury finishes his Revolution in eighty eight Days, Venus in two hundred and twenty four and some Hours, the Earth, as you already know, in a Year, Mars in about two Years, Jupiter in almost twelve, and Saturn in about twenty nine Years and a Half.

There is so great a Dependance and Relation between the periodical Revolutions of the Planets and their Distan <150> ces from the Sun, that if we know the Distances of any two (of the Earth and Jupiter for Instance) and the periodical Time of one, (suppose of the Earth) by a certain Rule we may find the periodical Time of the other.

I should have a much clearer Idea of your Meaning, said the Marchioness, if I had first read Fontenelle's Plurality of Worlds to convince me of the Motion and Agility of the Earth. As you are now so great a Proficient in Philosophy, I replied, you must seek the true Demonstration of this Motion in England.

There have been observed certain Appearances in the Stars which some imagined to be Consequences of this Motion, but others who examined it more strictly, have discovered these Appearances to be contrary to the Laws of such a Motion. The Motion of Light which is a considerable Time in coming from the Stars to us must strangely vary their Appearances, and ought to be considered in Conjunction with the Earth's Motion round the Sun, in order <151> to give a right Decision of the Question.

The Sagacity of the English Philosophers has in Fact united these two Motions, and by this Means they explain these surprizing and various Appearances which in any other System are absolutely inexplicable. And thus we have attained to the Certainty of a Thing proved a thousand Ways, but never strictly demonstrated by any.

The five Planets in whose Number we may safely replace our Earth, are called Primary, to distinguish them from other subaltern Planets which revolve round a Primary as the Moon does round our Earth, the four Satellites of Jupiter round that Planet, and five round Saturn; and these Sabalterns are called Secondaries. These last agree with their Primaries in observing this Order, that the nearest compleat their Orbit in a less, and the most distant in a greater Time; and they keep this Law with the same Exactness and the same Relation as their Primaries do.

Another Law in which the primary and secondary Planets agree is, that they <152> do not describe equal Parts of Orbits in equal Times, but such Parts of Orbits as to make their Areas equal. That you may better understand this other Law of their Motion, you are to suppose that the Orbit of a primary Planet is very near a Circle which the Sun is not placed directly in the Center of, but a little on one Side. Imagine a Line to be drawn from that Point of the Orbit where the Planet now is, to the Sun, and another Line to be drawn from that Point where the Planet will be twenty four Hours hence. The Space contained betwixt the two Lines drawn to the Sun, and that Part of the Orbit which the Planet has described in twenty four Hours, is called the Area, and will be equal to another such Area, that the Planet will describe in twenty four Hours more; and thus in equal Times the Areas will be always equal.

The Areas then, as Astronomers express it, are proportional to the Times. Thus if instead of twenty four Hours we put twelve, which is one Half, an Area described in those twelve Hours will be <153> only one Half of the Area described in the 24; and so, if we take a third or fourth part of the Time, the Areas described in that third or fourth Part, will be the third or fourth of those described in the first Time; and if that Time be doubled, the Area described in it will be doubled likewise, and so on. This Law which the primary Planets observe with Regard to the Sun, the secondary Planets observe with Regard to the Primary, round which they revolve; and this Primary is the same to its Satellites, as the Sun is to the Planets of the first Order.

I am extremely pleased, said the Marchioness, with the Agreement between these two Sorts of Planets. I regard the Sun as Monarch of the immense planetary Realm, in which the primary Planets are the Grandees and Nobles; some of these possess certain Districts where they exercise the same Jurisdiction in little which their Sovereign does in great, but all shew their Dependance by revolving round him alone.

Our Earth is in Possession of a little Province, where she exacts Obedience <154> from the Moon; and tho' she cannot vie with Jupiter or Saturn who have a greater Number of Dependants, she is certainly superior to Mars, Mercury and Venus, who have none.

Your Similitude, answered I, would be juster in the Cartesian System of Vortices where these Jurisdictions seem very well established; and it would be the more so, because this philosophical Poetry is fond of imbellishing itself with Comparisons and Similies, and sometimes makes them even serve for Reasons. But the two Laws I have mentioned to you, will not suffer it. Indeed it is Pity we are obliged to abandon these Vortices, that present the Mind with so clear, so natural and simple an Idea. The Planets, say the Cartesians, revolve round the Sun, because a certain Fluid in which they are immersed turns round too, and carries them with it like little Skiffs forced along by the Current of a River. The secondary Planets revolve round their Primaries for the same Reason.

Nothing seems to be more evident than this. But the Misfortune is that <155> these Planets are not contented with barely turning round, but will do it by certain inviolable Laws that intirely destroy all these imaginary Systems.

Either these two Laws cannot agree with the Vortices, or agree so ill, notwithstanding all the Efforts to that Purpose, that one of their most illustrious Defenders confesses, after all he has done in their Support, he doubted whether those who refuse to admit them would not be confirmed in their Opinion by the very Manner in which he has endeavoured to defeat it. Besides these Vortices are pressed by so many other insuperable Difficulties, that Heaven itself seems to have conspired in the Destruction of this fine Poem.

Far be it from us, replied the Marchioness, to oppose the Decrees of Heaven. On the other Hand, I cannot reconcile myself to the Idea of a Poem in Philosophy. What is this philosophical Poetry to which I cannot assign a Place in my Thoughts? It ought to content itself with influencing the Passions of Men, but it has nothing to do with the single Passion of Philosophers, which is Truth.


The Newtonian Principles, answered I, have inspired you with very rigid Sentiments. But this Poetry, that thinks itself too much limited in the vast Field of human Passions, shall give you no further Trouble. Comets, the most declared Enemies in all Heaven to their Vortices, will I believe be sufficient to overthrow it, for they seem to be made expressly for the Destruction of Systems.

It has been established, by Virtue of I know not what, but however the Philosophers have very readily believed, that the Matter of the Heavens was incorruptible, and that every thing there flourished in a perpetual Youth, insusceptible of the Changes and Vicissitudes which happen here below. The Comets appear at first almost naked, but in their Approach to the Sun are cloathed with a formidable Tail, of which they gradually divert themselves as they recede from him, and so return back naked as they came. And in this Manner is the System of the Incorruptibility of Heavenly Regions, in great Danger of being overthrown by these impertinent Comets. And this perhaps <157> is the Reason why they were degraded from their Celestial Seat as worthless Meteors formed by the Vapours and Exhalations of our lower World. But they would not remain long there; for besides many ancient Philosophers who considered them not as one of the transient, but durable Works of Nature, the Astronomers who must have their Share in a Thing above us, assure us, that they are very distant from the Earth, and some of them farther off than the Sun himself. These Comets, said the Marchioness, are very bad Omens to Systems, if not to crowned Heads. These were not all the Troubles, said I, which they gave Philosophers. When they were placed among the celestial Bodies, they could not agree with that Solidity which had been granted to the Heavens upon the Word of Aristotle; in order therefore to avoid their demolishing and breaking to Pieces the whole Universe in their Passage through these Aristotelian Heavens, it was necessary to resolve upon making these last fluid.

When the Heavens were thus made fluid they became Vortices, against <158> which these factious Comets renewed their Enmity with more Violence than ever, to destroy an agreeable Imagination received by the World with so much Applause, and which was defective in nothing but Truth.

Some Comets have made no Difficulty to cross the Orbits of all the Planets, proceeding almost directly from the superior Part of the Vortex to the Sun; others have moved in a Course absolutely contrary to that of the Planets, without meeting either in the first or second Case, any Resistance in their Motion, which must necessarily have happened, if there was a certain Matter that whirl'd round the Sun, and this at several Distances from him with the same Rapidity as the Planets supposed to swim in those Vortices. Their Motion would have been so weakened, that turning round the same Way as the Planets do, they would soon have yielded to the irresistible Force of the Vortices, not unlike the unfortunate Barks, which guided by an unskilful Pilot or a malignant Star, are Ship-wreck'd in the horrible Cata <159> racts of the Chinese Rivers, notwithstanding all the struggling they can make to the contrary.

In short, these Comets have in every Instance acted directly opposite to the Laws of Vortices. So that to rescue them from the continual Injuries they receive from these implacable Enemies which lay hold on every Occasion to commit all Sorts of Hostility and Impertinence, I see no other Remedy than to destroy and banish these unhappy Fluids for ever from the System of the World.

Your Expedient, replied the Marchioness, is no less violent than that sometimes used in War, when one Party destroy and ruin a Country which they cannot defend against the Enemy. And thus they make the same Sacrifice to their Weakness as you do to Truth. This is a Sacrifice at which I cannot be displeased, especially as it puts me into a better Capacity of listening with more Tranquility to the new Principle upon which the celestial System is built.

Sir Isaac Newton, continued I, founded his Scheme in Geometry, which we <160> may call his native Country. He began with demonstrating that if a Body in Motion is attracted towards a Point either moveable or immoveable, it will describe about this Point equal Areas in equal Times, and in general, that the Areas will be proportional to the Times; and on the contrary, if a Body describes round a moveable or immoveable Point Areas proportional to the Times, it will be attracted towards that Point, that is, the Body will have such a Tendency towards the Point, that if every other Motion which impels it a different Way should cease, the Body would directly unite itself to that Point, just as Bodies here below, when left to themselves, fall directly upon the Earth.

This Principle, interrupted the Marchioness, is equally applicable both to the primary and secondary Planets. Each of these describe Areas proportional to the Times, round the Point about which they turn (if the Sun, our Earth and Jupiter may be termed Points). The primary Planets then are attracted by the Sun, and the secondary <161> by their respective Primaries about which they revolve. Is not this a necessary Consequence? It is without Dispute absolutely necessary, answered I. But remember, Madam, this is a Deduction of your own. This Punishment is just, since you have made so much Difficulty to admit the Principle of Attraction.

You say then that there is a Force in the Sun which attracts the Planets to him, and after the same Manner, a Force in the Planets that attracts the Satellites, and this attractive Force joined with another by which they all move from West to East, is the Reason why the first revolve round the Sun, and the others round their Primaries, in a certain Order.

The Ancients, in order to explain this difficult Phænomenon, built solid Heavens and created Intelligences to put them in Motion; on the other Hand, Des Cartes had imbarassed the whole Universe with the great and magnificent Apparatus of his Vortices. But after all, the Motion of the heavenly Bodies is by Sir Isaac Newton reduced to the most simple yet the most noble <162> Phænomenon in the World, which has been rendered much more familiar in Europe, than is agreeable to some Persons. In short it is no more than that of a Bullet, which would of itself proceed in a direct Line, if the attractive Force of the Earth did not oblige it to move in a Curve. The Bullet very soon falls to the Earth, because the greatest Force we can possibly give it, is but little when compared to the vast Extent of this Globe. If it were possible for human Weakness to throw one from hence beyond Peru, it is demonstrated that we should acquire a new Satellite; it would like the Moon revolve about our Earth, only its Motion being necessarily very soon weakened from the continual Resistance of the Air, while the gravitating Force would lose nothing of its Strength, this new Moon would at last fall and destroy every thing it lighted on, after we had heard it make a horrible hissing over our Heads.

All this you explain in two Sentences; an evident Proof of the Significancy of <163> a Lady's Words. What you have said is certainly a great deal, but not all. It still remains to know by what Law this attractive Force acts, that is, whether it be the same at all Distances from the Sun, or whether it grows weaker in Proportion as the Distance is greater. I will resolve you this Question, the Marchioness replied, when you have furnished me with as many Hints for that Purpose as you did when I told you that the Planets are attracted by the Sun; and I hope you will afterwards make as genteel a Commentary upon me as you did in the other Case.

That Law, answered I, of describing Areas proportional to the Times observed by every particular Planet, furnished Sir Isaac Newton with Means to discover the attractive Force of the Sun; and that other Law which they observe, of describing their Orbits in a greater Time in Proportion as they are at a greater Distance from the Sun, and that with a certain Relation between these Times and their Distances, helped him to find out that the attractive Force di <164> minishes as the Distance from the Sun increases.

The attractive Force diminishes with this Proportion, that it is always so much less as the Square of the Number which expresses the Distance from the Sun is greater. In order to understand this Cyphering (which perhaps at first Sight may appear very formidable) it is necessary to acquaint you that the Square of any Number is nothing but that same Number multiplied by itself, as four for Instance is the Square of two, because twice two makes four, that is two multiplied by itself gives four.

I may now safely venture to propose you a Problem, that as you have lately explained the Phænomenon of Natural Philosophy, you may now undertake the Solution of mathematical Problems: After this I do not see what you can do better than to shew some Sort of Gratitude, and discover the Truth to him who has conducted you into its most abstruse and retired Paths.

The Problem I shall propose to you is this: Suppose the Earth's Distance <165> from the Sun to be one, and Jupiter's Distance from him to be about five, (considered with Respect to that of the Earth,) the Question is to know how much the Sun's attractive Force will be diminished at the Distance of Jupiter?

Give me a little Time to consider, answered she with some Impatience, for the Solution of a Problem is no trifling Affair. You have informed me that the attractive Force is so much less, as the Square of that Number which expresses the Distance is greater. The Square of one which is the Earth's Distance from the Sun, is one. And at the distance one, answered I, the Forces is supposed to be one, and the Question you are to resolve is, How much that Force will be diminished at the Distance of Jupiter from the Sun which is five. The Square of five, answered she with great Quickness, is twenty five. If the attractive Force must be so much less as this Square is greater, it follows that in Jupiter it is twenty five times less than in the Earth. Is not this the Solution of your Problem? And may not I, like Ar <166> chimedes, run crying about, I have found it, I have found it?

Yes certainly, answered I, but not in the same Circumstance as he did, when his Impatience was so great as to make him run precipitately out of the Bath. The Mathematicians ought rather to act as Pythagoras did upon the Discovery of a certain Truth, and sacrifice a Hecatomb to solemnize this Day which gives them Liberty to imbellish and brighten their gloomy Catalogue by your Name.

The Law that the attractive Force observes of growing weaker at various Distances from the Sun, is the very same to which all other Qualities that flow from Bodies, are subject; as Smell, Sound, Heat, and, which most nearly concerns us, Light. Thus when you believe yourself to have solved only one Problem, you have in reality solved two. Is then, replied she, the Light of the Sun as well as his Attraction twenty five times less in Jupiter than with us? The very same Number, answered I, serves equally for both. After the same <167> Manner you will find that the Attraction, the Light and Heat of the Sun, must be ninety times less in Saturn than with us. The Twilight of our farthest Laplanders would be there the finest Summer Days, and in the most raging Dog-star of that Planet our Seas hardened with perpetual Ice, instead of swift sailing Vessels, would groan under the Weight of heavy Chariots; whereas in Mercury they would even in the Depth of Winter be dissipated into thin Vapors (occasioned by his extreme Proximity to the Sun) and would thus leave their Bottom dry, and present to Pilots a horrid Gulf, a dreadful View of the Terrors of the Deep; and to Naturalists a beautiful Scene that would furnish them Materials to enrich their Musæums.

You see, answered she smiling, how many fine Discoveries I have made without perceiving any thing of it. It is however true, that great Affairs are generally brought to pass we know not how, and we are at last amazed to find them effected.

In human Affairs, answered I, it is <168> ascribed to their good Fortune, if the Cæsars and Alexanders after proposing only one End, acquire another which they never dream'd on. It often happens, that those very Persons who are called fortunate gain that Name by some Events very different from what they intended. The Inventer of Gunpowder, 'tis probably, proposed a quite different End to his Studies than the Discovery of a Secret to destroy Mankind with the greater Facility; and that Person who found a new World sought nothing but a more expeditious Way to the richest Part of the old.

On the other Hand, in true Natural Philosophy and Geometry, the Cæsars and Alexanders are more common. It is very seldom that we find only what we sought. The Discovery of one Truth frequently produces many others which appear in Spite, as it were, of those who seemed to disregard them. Any one who carefully seeks that Law by which the attractive Force ought to act at various Distances, will at the same Time discover that universal Law by <169> which all the Qualities which flow from Bodies are governed. Natural Philosophy afterwards illustrates this general Truth with peculiar Experiments, and in some measure translates the abstruse Hieroglyphics of the learned Tongue into vulgar Language. That Decrease of the attractive Force which immediately concerns Light, is demonstrated by a very easy Experiment, which we may try this Evening, if you are not already sufficiently tired with Philosophy and Experiments.

Suppose one single Candle to be placed in a Room, and recede from it to such a Distance as not to be able to distinguish the Characters of a Book or a Letter, unless perhaps it were a Billetdoux, which may be read at any Distance.

Place yourself afterwards at a Distance twice as far from the Candle as you were at first. In this Situation the Force of the Light must be, according to the established Law, four times less than it was at the first Distance. The Letter then cannot be read with the <170> same Distinctness as it was at first, unless the Light be quadrupled: That is, the Law requires that in the Proportion as the Light grows weaker, the Square of the Distance must increase. And this proves the Experiment to be true; for the Letter at the second Distance is then only read with the same Distinctness as at the first, when three more Candles are added to the single one, or, in other Words, when the Light is quadrupled.

Considering how very easily People are apt to forget those Objects in their Absence which made the greatest Impressions upon their Mind when present, I cannot help thinking, said the Marchioness, that this Proportion in the Squares of the Distances of Places, or rather of Times, is observed even in Love. Thus after eight Days Absence Love becomes sixty four times less than it was the first Day, and according to this Proportion it must soon be entirely obliterated; I fancy there will be found, especially in the present Age, very few Experiments to the contrary. I believe, said I, that both Sexes are included in this <171> Theorem, which seems rather to follow the Cubes of the Times, which is certainly more convenient, and requires only four Days for an intire Oblivion. But, in general, I believe we may without Scruple establish the Proportion of the Squares, for eight Days are commonly enough to cure the most vehement Passion. You along have Power to reverse this Theorem, and make the Remembrance of you, and with that, a Desire of seeing you, instead of diminishing, increase according to the Squares, or rather the Cubes of the Times. No! no! said the Marchioness, Gallantry must never destroy a Theorem. I am willing to enter into the general Rule, and shall think myself exceedingly happy if I have been able to establish any thing fixed and constant, in an Affair so inconsistent and wavering as Love. If Geometry, answered I, was permitted to get some Footing there, it would in a little Time produce Wonders. The Conclusions would be the most ready and elegant imaginable.

But to be serious, said she: Our Con <172> clusion in Natural Philosophy was, That the attractive Force of the Sun diminishes in Proportion as the Squares of the Distances increase. I suppose the attractive Force of the Planets will follow the same Proportion with Regard to their Satellites. The Satellites, answered I, which turn about any Planet, observe the same Relation between the Distances and the Times of their Revolutions, as the Planets themselves do that turn about the Sun. This is evident in Jupiter and Saturn, who have more than one Satellite, and consequently the Law of their attractive Force will be the same as that of the Sun.

In the Earth, who has only one single Satellite to her Share, this is not altogether so evident. But what Reason is there why it should not be the same in one as in the other?

If we had another Satellite revolving about our Earth at a Distance different from that of the Moon, it would discover whether the attractive Force of the Earth observes the same Law as that of the Sun Jupiter and Saturn. This <173> Defect however is supplied by the Bodies which we see every Day fall upon the Surface of the Earth; for we are to believe that the Force which would make the Moon fall if she lost her Motion from West to East, is the very same that makes Bodies here below fall upon the Surface of the Earth when they are left to themselves: For since it is demonstrated that the Earth has an attractive Force, it is evident we must in this Force seek the Cause of Gravity, another Phænomenon, which the Vortices have been as unsuccessful in the Explication of, as in that of the planetary Motions.

If we could raise Bodies from the Earth to very considerable Distances (compared with that at which we stand from the Center which is very great,) we should see the Force of Gravity prodigiously diminished in them. A Man of War of a hundred Guns, for whose Formation a whole Forest was cut down, and a whole Mine exhausted, would be overset by the slightest Breeze of a Zephyr. The famous Stone-Henge <174> upon Salisbury Plain, the fruitful Source of Fables both to the Learned and Ignorant, those Colossian Heaps which are held together by the Force of Gravity, would be no more than Houses built of Card. The Velocity in the Fall of heavy Bodies would be considerably retarded. Bombs, those artificial Thunders, would not be more terrible than so many Flakes of Snow. But these Experiments are impracticable; one of the greatest Distances we can attain is Pike Teneriff, which is only about three Miles perpendicularly high. Besides the Air would be too thin for Respiration, and the Cold, which must be exceedingly sharp at a greater Height, would render any Experiment fatal to the Philosopher who had the Courage to undertake it.

Nature, replied the Marchioness, has in this Case denied us the Means of being compleat Newtonians. She has here confined us within the Bounds of Probability. If the attractive Force observes a certain Law in the Sun, Jupiter and Saturn, why should not the same Force <175> observe it here on our Earth? In this Point, answered I, we have no Reason to complain; we have no Need of higher Mountains, and a different Constitution of Air, in the present Case. All these Things, and the Defect of another Moon, are, as I before observed, supplied by the Bodies which fall upon the Surface of the Earth. We may compare these Bodies to the Moon herself; and thus instead of Probability we shall have Evidence, and be even in this Point good Newtonians.

It has been deduced from Observation, that if the Moon should lose her Motion, and fall towards the Earth, that Force which would set her a falling would be three thousand six hundred times less than the Force which makes Bodies fall upon the Surface of the Earth. You see how well this agrees with our Principle. The Moon is distant from the Center of the Earth (where the attractive Force chiefly resides) sixty times as far as these Bodies: The Square of sixty is 3600: The Attraction then of the Earth to the Moon is diminished in the same Propor <176> tion as the Square of the Distance is increased; and this is exactly agreeable to the established Law in Jupiter, Saturn, and the Sun.

If the Moon should happen to fall upon the Earth, replied the Marchioness, it would present a fine and agreeable Sight to the Newtonians: They would certainly have neither Curiosity, Eyes, nor Calculations for any thing else. This might very easily happen, answered I, if every thing was Body, as the Cartesians affirm; and those ancient Gauls, who were apprehensive the Heavens might one Day or other fall upon their Heads, would have some Reason to fear it in the System of their own Des Cartes: For it has been demonstrated that if this Planet was to move in a Place absolutely filled with Matter, without the least empty Space (let this Matter be supposed ever so subtile, fluid and æthereal) her Motion from West to East would be so retarded that it would soon grow weaker, and at length totally fail. And thus yielding to the Force of Gravity she would fall precipitately from <177> Heaven to Earth, and we should no more behold her that triform Goddess we before admired, but a Stranger banished from the most shining of her three Kingdoms, and no longer the Ornament of Heaven amidst the friendly Silence of the Night.

The other Planets would undergo the same Fate if they moved in a Plenum. These would all, some sooner and others later, fall into the sun, and supply that immense Volcano with a greater Quantity of Matter. He would then reign the Sovereign of a depopulated Empire: His animating Light would shine in vain: Not a single Planet would be left to partake his pleasing Influence, nor receive from him the Seasons and the Day; for both the Comets and we with our Moon should be stifled in him, if we met with any Obstacle in our æthereal Road. This would be a new Punishment to an Age fruitful of Crimes, in the System of that English Writer[1] <178> who makes the glorious Body of the Sun the Mansion of Grief, and the Seat of eternal Despair.

I assure you, for my part, I should be one of the first, continued I, who would run to see the Moon fall upon the Earth. What an agreeable Spectacle would it be to see, in Proportion as she approached to us, that Face, that Mouth and Nose, which we discover in her rather by our Imagination than our Eyes, gradually transformed into great Mountains, Vallies, Plains and the like, which must certainly fill the Vulgar with great Astonishment: Nay even Philosophers themselves, who can never sufficiently master those two great Enemies of Reason, Fancy and Prejudice, could not help looking on this Phænomenon without some Degree of Surprize. As she approached still nearer, said the Marchioness, should not we descry the Sights of Lovers, Dedications to Princes, Courtiers Promises, Vials filled with the Judgment of our Sages, and all the other lost Things which Ariosto places there? You have <179> not read the Plurality of Worlds, answered I, and therefore are not capable of seeing ht greatest Curiosities in that Planet; since you are not yet acquainted with the Force of a Why not, which peoples the whole Universe.

But one Thing which I should take great Pleasure in observing, if the Moon should really happen to fall, is, what Treatment the Earth would give the Moon in going to receive her.

What, replied the Marchioness, is it a Point of Ceremony established among the Planets, that if a Satellite should fall upon its Primary, the latter must go meet it and shorten its Way? This Ceremony, answered I, is founded upon mutual and reciprocal Attraction. If the Earth attracts the Moon, why should not the Moon attract the Earth? The Attraction which the Earth exercises upon the Moon is lodged in that Matter of which the Earth is composed, why then should not the Matter of which the Moon is composed exercise its attractive Force upon the Earth, since all Matter is entirely the same, and <180> only differently modified in different Bodies? Besides Action, as the Philosophers express it, is always equal to Reaction. You cannot press this Table with your Finger, but it will be equally re-pressed by the Table. Thus if two little Gondolas made of Cork (in one of which is placed a Loadstone, and in the other a Piece of Iron) are set a floating upon the Water near each other, the Loadstone will run as fast towards the Iron as the Iron does towards the Loadstone; and if either of them be hindered, that left at Liberty will spring towards the other, which could not happen unless the Iron attracted the Loadstone as much as the Loadstone itself does the Iron, or, in short, unless their Attraction were reciprocal.

I perceive, said the Marchioness, what will be the Conclusion of all this. The Sun attracts the Planets, and consequently the Planets attract the Sun: The Secondaries attract each other, are all attracted by the Sun, and all attract him. Does not this Multiplicity, this <181> Chaos of Attractions perplex not only me but the System itself too? No, Madam, answered I, it happens in this just as in the new Geometry (of which I was speaking to you the other Day) in which all those innumerable Orders of infinitely small Quantities, instead of perplexing, render it more subtile and perfect.

This mutual Attraction, diffused through the Universe and all its Parts, retains the wandring Planets in their Orbits, and connects all Bodies, the Earth, and us ourselves, by strong tho' invisible Ties, and regulates and tempers every Motion in such a Manner, that its Existence and inresistible Laws are every Instant apparent.

In thinking on this reciprocal Attraction, said the Marchioness, something comes into my mind, which I dare not however propose as an Objection to a System which the Philosophers themselves cannot venture to oppose. It appears to me that if there really was such a mutual Attraction, we must see the Effects of it in those <182> Bodies which surround us, if not every Instant, at least very often, just as we discover from their Gravity the Effects of that general Attraction which the Earth exercises upon them.

When any little light Body, as a Feather for Instance, be placed near a Palace, a Hill, or any Thing whose Attraction is very strong, why do not we see it presently obey the Force which attracts it, and move, as it should do, towards the Palace or Hill? When our Mind is possessed by a very strong Passion, answered I, what is the Reason we do not feel the weaker, unless it be that the strong Passion attracts the whole Soul, so as not to suffer the weaker to make any Impression upon it, and thus we become insensible to all the rest which are not in themselves either light or weak?

The furious Passion of Phœdra for Hippolytus in Racine does not permit her to feel that strongest Passion in the Fair Sex, the Desire of Beauty. Her Ornaments and her whole Dress are in that Disorder which perhaps neither the Absence <183> nor Death of her Theseus could ever have produced in them.

I comprehend you, replied the Marchioness, notwithstanding your parabolical Method of explaining this Point. The very great Attraction which Bodies feel from the Earth, if I may use the Expression, render them insensible to that of the other Bodies which surround them. Bodies, answered I, attract only in Proportion to the Quantity of Matter they contain. I make no Scruple of using mathematical Terms to you, for to use any other would be an Affront to a Person who has given the Solution of a Problem.

Thus a Ball of Gold, besides many other Advantages, has a greater attractive Power than that of Wood, because it has a greater Weight; and if the first be a hundred times heavier than the last, that is, if it contains a hundred times greater Quantity of Matter, it will have its attractive Power a hundred times stronger.

Now the Attraction of this great Ball upon which we stand is diffused on <184> all Sides, and draws every Thing to it with an immense Force, and by that Means hinders us from perceiving the Effects of that particular Force, which the little Balls by which we are surrounded exercise upon each other.

A Globe of the same Density with the Earth, and of a Foot Diameter, attracts any little Body placed near its Superficies, twenty Million times less than the Earth does. The Attraction of the highest Mountains as that of Pike Teneriff, Ararat, or even the Apennine, notwithstanding the pompous Description made of it, is absolutely imperceptible.

But it is not so with the Effects of the Lunar Attraction upon this vast Collection of Waters that some Philosopher made the first Principle of Things, which by the easy Method of Navigation joins the most distant Countries, and transports to us from another World those Balms and Aromatics which season the European Entertainments. You seem, replied the Marchioness, to have a very <185> lively Sense of our Obligations to the Ocean, and the Advantages we receive from it. But has not the Philosopher amidst these Entertainments forgot the Attraction of the Moon? The Ocean, answered I, every Day shews us the Effects of this Attraction, which extends its Empire throughout all Nature. The Ebbing and Flowing of the Sea, a Phænomenon, which in the most polished Age of Greece, Alexander the Great took as a Mark of the Divine Displeasure, and with which the Romans in the Gold Age of Julius Cæsar were but very little acquainted, is only a Consequence of that Attraction the Moon exercises upon the fluid and yielding Part of our Globe. Chapelle in his celebrated Voyage, that Model of polite and agreeable Wit, believed that no one but a Water-God could penetrate into its Cause.

This God relates to him, that when Neptune was made Governor of the Sea, the Rivers went to congratulate him. The Garonne upon this Occasion retained something of the haughty Temper of his Country, and his Compliments were not <186> so submissive as was necessary in addressing that Power who raises the Tempest by a single Nod, and with an I will ---- silences them to a profound Calm.

The Punishment which this arrogant River received for his Crime, was to be repelled twice a Day back to his Source. All the Rivers which flow twice a Day into the Ocean undergo the same Fate. But why, answered the Marchioness, should the other Rivers who were not guilty of this Gascon Behaviour suffer the same Punishment as the Garonne? If it was permitted to make Objections to Superior Powers, I would humbly propose this to Chapelle's Neptune.

You may make Objections equally strong, answered I, to all the human Systems that have been made to explain this Phænomenon. Some affirmed the Respiration of this vast Animal the Earth and the Sea to be the Reason of it, others a great Gulf in the Northern Ocean near Norway, which emits a vast Quantity of Water and afterwards swallows it up again, a most fatal Circumstance both for the Fishes and <187> Philosophy which have the Misfortune to be plunged in its Deeps. The ancient Chinese, who supposed their little Country of four Leagues Extent, to be the Universe itself, asserted that two great Nations descended from a certain Princess, the one inhabiting the Mountains, and the other the Sea-shore, had frequent Wars with each other, and this was the Reason of the Flux and Reflux according as either of the Combatants was drove back towards the Mountains or the Sea.

Such perhaps was the Infancy of Philosophy among all, even the most sagacious People. The Explication of Des Cartes which was produced in a later Age of the World is ingenious enough to render it agreeable, but not accurate enough to be true. The same English Philosopher who cast an impenetrable Obscurity over Vision, by involving it in his contractile and expansive Forces, has endeavoured to involve this Phænomenon also in the same unintelligable Terms. This gloomy Fancy diffused itself like an universal Contagion <188> over the whole Face of Things, and infected the whole System of Natural Philosophy.

According to his Opinion, the most simple and evident Method of explaining the Tides, is by ascribing their Cause to the contractive Forces of the Earth and Moon, by which the one raises and the other depressed the Water. To these he unites the expansive Force of the Sun, which, tho' always contrary to the contractive, yet upon this Occasion must act in Concert with that of the Moon. These unintelligible Terms, which have not so much as the Fashion to support them, can imply nothing in the Author but a vehement and vain Desire of giving his Name to a new System of Errors. These Philosophers, said she, appear to me the Priests of Chapelle's Divinity: Their Explications at once discover both the Temerity and Weakness of their Philosophy. Our's, answered I, takes delight in Difficulties, and comes off in Triumph; amidst the Thorns we are sure at least to meet with Roses.


That Portion of the Water immediately under the Moon and nearest to her, must be more strongly attracted than the rest which she looks obliquely on, and which is at the greatest Distance from her. The Ocean then must flow together from all Parts, and be heaped into a Mountain of Waters whose Summit will be under the Moon herself.

The Earth itself is a little attracted by the Moon, but that Part of the Water which is directly opposite to the Part under the Moon is least attracted, because it is at the greatest Distance.

This Part then will be as it were forsaken by the Earth which a little follows the Attraction of the Moon, and will there form the Summit of another watery Mountain, and thus there will be two, the one totally opposite to the other.

The Ocean then must swell, and in some measure lengthen itself in that Part where the Moon is, and in the Part opposite to her, and thus from the Figure of an Apple be changed to that <190> of a Lemon, whose Extremities will always follow the Moon in her diurnal Course; so that the Water will be at one Time depressed, and at another raised, both in the same Place.

In every Part of the Ocean there will be two Tides, during the Time which the Moon employs in returning to the same Point of the Heavens. When she is at the Meridian the Waters must be raised, and when she sets, depressed; another Elevation when she is at the Meridian of the Antipodes, and another Depression when she rises.

All this must unavoidably happen if the whole Earth was covered with deep Waters, and they immediately obeyed the attractive Force of the Moon. But because there is some Time necessary for the accumulating the Waters, and their Course is interrupted by Shores, Straights, Islands, and the like, there are some Irregularities in the Tides.

These Irregularities however are not so great, but twice in every twenty five Hours (which is nearly the Time that the Moon employs in her Return to the <191> Meridian) we see the Vessels laden with the Riches of the Universe go up with the Tide along the Silver Waves of the Thames, and twice descend with the Reflux to go in Quest of new Treasures. And this Advantage, which in the System of Chapelle was inflicted as a Punishment, all the Rivers which flow into the Ocean enjoy.

Have our Mediteranean Rivers, said the Marchioness, ever offended the Moon, that they are not suffered to enjoy the same Advantage? Perhaps they have affronted her as the Garonne did Neptune.

The Straight, answered I, by which the Mediteranean has a Communication with the Ocean, is too narrow for so great a Sea, and is disadvantageously situated, (since it looks towards the West) to receive the great Flood of the Ocean that follows the Moon from East to West.

On the other hand, the Tide which is formed in the Mediteranean itself is too much interrupted with Islands, Shores and Straights, for the Flux and <192> Reflux to be very considerable. In the Adriatic, on the contrary, it is more sensible than in any other Place, because the Sea is very narrow, just as the Motion of a River is most apparent and seems most rapid when confined between the Arches of a Bridge. In our fine City founded by the Gods upon the Ocean, the Vicissitudes of the Flux and Reflux carry the fluctuating Gondolas from one Side to another, while the Gondolier sits at his Ease and singing to the agreeable Light of the Moon, teaches the Sea-nymphs the Flight of Erminia, or Rinaldo's Love.

The Changes of the Tydes are still less perceptible in the Baltic-Sea, the Mediterranean of the North; for this Sea bordering upon the Frozen Regions of the Pole, and very distant from the Course of the Moon, seems more adapted to Ice and Rocks than to Warmth and Attraction.

In the Shores of the southern and oriental Oceans, to Japan and China, the Tyde is very considerable by reason of the Extent of the Seas; and <193> in our Ocean its Effects are incredibly prodigious. There are Coasts near Dunkirk, where the Sea draws back for the Space of several Miles, and afterwards on a sudden returns and overflows the same Space again, alternately covering and disclosing those Sands so very suspicious to Sailors, not without sometimes disturbing the Ladies in that Country, who venture to walk upon the Margin of the Sea, whose very Shores are faithless and deceitful. These are a Sort of natural Sea Fights, where at some Parts in the Day two Armies may engage on dry Land, and at others, two Fleets, at least such as those of the Ancients were.

In some Rivers the Tide rises to more than fifty Foot high, especially if the Sun and Moon act in Conjunction to render the Flood considerable.

Tho' the Moon may be regarded as Empress of the Ocean, the Sun however has his Share in it. Notwithstanding he is at a greater Distance from the Earth than the Moon is, yet in Return he is so much greater, that it is not pro <194> per he should remain idle, but lend his Part to the Production of the Tides. The rest of the heavenly Bodies have no sensible Influence in this Case, the vast Distance they are separated from us renders them too small.

When the Moon is in her Quadratures the Tydes are less than at any other Times in the Month. The Reason of this is, because the attractive Forces of the Sun and Moon crossing each other, are as contrary as is possible to the Swelling of the Sea in the same Situation.

On the other Hand, when the Moon is new or full, she is in the same direct Line with the Sun in respect to the Earth, their Forces are united, and then the Tides are highest.

It is to be observed however, that the Motion begun in the Waters and retained for some Time within them, must be some Days after the new or full Moon, before it produces the greatest Raising of the Sea. So in Summer, the Heat of Mid-day that remains in the Air, being added to the follow <195> ing Heat, less intense in itself, is the Reason why we have not so great need of the Assistance of the Fan to supply us with fresh Air at Noon itself, as we have some Hours after.

The greatest of all the Tides happens at the new or full Moon of the Equinoxes; for besides the Conjunction of the solar and lunar Force, the Waters in this Case acquire a greater Agitation; but because the Sun (notwithstanding our freezing) is nearer to the Earth in Winter than in Summer, these great Tides do not happen at the precise Time of the Equinoxes, but a little before the Vernal, and something later than the Autumnal, that is, in the Month of February and October.

In Mercury, Venus, and Mars the Tides are governed by the Sun alone, tho' they must be almost insensible in Mars, because that Planet is at so great a Distance from him. In Jupiter and Saturn the Distance from the Sun is so very great, that he can have absolutely no Influence over the Tides. They will confound each other accord <196> ing to the Caprice of the Satellites, the great Number of which will render them very irregular.

If we knew the Time of Saturn's Rotation as we do that of Jupiter's, and were as well acquainted with the Geography of both, and the Quantity of Matter contained in their Moons, as we are with their Distances and Revolutions, we might conjecture the Quantity and Period of their Tides, and send Tables of them to their Pilots. Thus are we again transported by Attraction into Heaven, to vast and remote Worlds where this powerful Quality holds its most conspicuous and shining Seat.

By the Help of this, replied the Marchioness, we can travel thousands of Miles in an Instant, and are well recompensed by innumerable great and shining Truths. A French Author, I answered, a zealous Propagator of this System, like us transported by Attraction to these distant Worlds, thinks with great Probability, that the Moons of Jupiter and Saturn as well as our had once been Comets, which passing near <197> enough to these Planets to remain confined within the Sphere of their Attraction, were constrained to revolve round them, and thus degraded from the Rank of Primary to that of Secondary Planets.

Saturn has obtained so advantageous a Situation as to make him the most happy in the Number of his Conquests. For the same Reason he was able to acquire a fine Ring that incompasses his Body, and which was formerly a Comet that unhappily passed too near him.

This Saturn, replied the Marchioness, must be very terrible to the Comets that approach a little too close to him. He must be the same to them as the Cabo tormentoso, to which Avarice afterwards gave the Name of Good Hope, was to the Portugueze It must, I fancy, have been a very agreeable Sight to see Saturn at once adorn and enrich himself with a splendid Ring, while the poor Comet was forced to pursue its Journey, spoiled of the Honour of its shining Train.

He robbed it of nothing, answered <198> I, but what it enjoyed at another's Expence, according to the Supposition of a certain French Author, who assures us that the Comet in crossing the Atmosphere of the Sun, had from thence stolen its Tail. But the Newtonian Opinion is, that the Tails of Comets are formed and composed of certain Vapours arising from the Comets themselves when they are near the Sun.

It is not a very great Advantage, replied the Marchioness, to be possessed of a System that supplies even the Imagination with the most pleasing Amusement, by those strange and surprising Evens which it renders possible? And all this, answered I, only by the Force that among us makes a Stone fall to the Earth. This Attraction, said she, appears to be the same in the Hand of Nature, as the Subject of a Composition in that of a skilful Musician. Be it never so simple, yet when he undertakes it he will diversify it a thousand Ways, and make it appear every Moment new; in short, he will find enough <199> in it to form the most harmonious Concert in the World.

Nature, continued I, wants no other Subject at once to regulate and vary those innumerable and vast planetary Systems, which probably revolve round the fixed Stars, those luminous and attractive Suns that chear the Night, and which we debase by giving them the Names of our miserable Heroes. But why should these Heroes, said the Marchioness, remain unmoved and fixt? If they have a mutual Attraction, how comes it to pass that they do not approach each other and run all together? You have, perhaps, some other Parable ready which only waited till I should propose a Difficulty. No Madam, answered I, unless you take it for a Parable when I tell you that this would exactly happen, if the Number of those Suns was not infinite. Those which are upon the Superficies of this immense Sphere of Suns would be united to those next them, because they would not have any thing to attract them a contrary Way, and by that Means keep them in their Orbits. And thus these <200> successively running into those next them, and these last into others, they would be all heaped together. By this Means in a little Time there would be in the whole Universe only one Sun of an enormous Size.

But what is the Number of these Suns? What are the Limits of their Sphere? Is not the Center of it throughout, and the Circumference in no Place? The Difficulty you have moved would be alone sufficient to induce us to multiply the Stars ad infinitum, even if we had not a thousand Reasons besides.

I am quite lost, replied the Marchioness, in this Infinity of Suns and planetary Worlds; pray let us return to our own. We are possessed of a System capable of diversifying it to Infinity, if we were so fond of Infinity as to desire it. And a System, answered I, which predicts and gives a Reason for the very smallest Irregularities that can happen. After Attraction and its Laws were established, how sublime a Geometry was requisite to find out what Path the Planets must keep in the Spa <201> ces of Heaven, and how much more sublime still to foresee exactly how much they would deviate from that Path in the Constitution of the present Systems? The Vastness of the Object renders general Rules difficult, and the Niceness of the Variations renders the Exceptions still more difficult.

The Sun who was esteemed immoveable in the Center of the System, and imagined himself exempt from any Irregularity, is found however to be subject to it; for the Attraction between Bodies is always mutual, and every Cause must have a correspondent Effect proportioned to its Activity. As the Planets and Sun reciprocally attract each other, he must be sensible of their Force; so that to speak with the utmost Rigour, he continually changes his, as they vary their Situation with Respect to him. After all our Speculations then, replied the Marchioness, to prove the Immoveability of the Sun, we are at last reduced to make him move again. Had it not been better, continued she with a certain malicious Smile, to adhere to <202> the common Opinion without giving ourselves all this Trouble? Do not you act like those Persons who after employing their Reason to divest themselves of popular Prejudices, are afterwards obliged to have Recourse to the same Reason to resume them, if they have an Inclination to live and converse with their fellow Mortals?

Our Case, answered I, is extremely different. The Question then was to make the Sun revolve about the Earth with such a Motion as to run a Million and a Half of Miles in one Day. But at present the Earth herself continues to revolve about the Sun, and he has no other Employment than to approach or recede a little sometimes towards one and sometimes towards another Side of the common Center of the whole System. This Motion is insensible in Astronomy, and is in Fact only a mathematical Subtlety with which I thought myself obliged to make you acquainted. If the Planets were all on one Side, you are sensible that their united Forces must act upon him with the greatest possible Strength, in order to draw him back to them <203> selves and make him recede from the Center of the System: They would not however attract him, considering the Enormity of his Bulk, and more than one of his Diameters. I am very willingly convinced of my Error, said she. The Sun, who notwithstanding his vast Size is subject to the general Force of Gravity, may serve for an Example to great Kings, whom neither the Extent of their Fortune, nor the Superiority of their Station can ever exempt from an Observation of the universal Laws of Humanity.

Our Moon, continued I, is at present subjected by Attraction to the minutest and most exact Calculations of Astronomers. Her very Irregularities, her Caprices are reduced to certain and constant Rules. Comets, those Enemies of Systems who made still greater Resistence to the Power of Numbers than the Moon herself, are at length obliged to revolve about the Sun. And tho' their Orbits are much more oblong than those of the Planets, yet they observe exactly the same Laws.


By Observations made upon their Appearances Philosophers have assigned what Orbits the Comets must run in this System, and these in Fact are the Orbits which they really have run, almost with the same Exactness as the other Planets, notwithstanding the imperfect Observations left us by the Ancients concerning Comets, the Moderns have ventured to predict the Return of some of them, in the same Manner as they do Eclipses. And indeed what is there which this System might not authorise? A Titian could easily judge of a Picture from a rough Draught.

The Prophecy of that Ancient is now fully accomplished, who even in his Time forsaw that Posterity would calculate the Periods and predict the Returns of these Bodies, these eternal Monuments of the Ignorance and Weakness of human Nature.

It is expected that the Comets which appeared in 1658 will return twenty three Years hence, and I hope we may flatter ourselves we shall observe it together, you still young, and I not very <205> old. You shall be the Urania to give a right Direction to my Telescope. What a Vicissitude of Things, replied the Marchioness, is there in this System! I am metamorphosed into a Urania, and supposed to be young at a Time when it becomes unpolite to talk about Years, and the Non-Appearance of a Comet rendered more fatal than its Appearance! It will appear too soon, answered I, to put us in Remembrance of our past Time, and our Attraction. In this Case, replied she, we may express ourselves contrary to the common Saying,

'Tis Expectation makes a Blessing dear.

'Tis at present indeed a great happiness to be an Astronomer. They at least expect nothing in vain, and how great a Pleasure do they owe to this System, which gives them a full Power over every Thing in that Heaven which is the Object of their Projects and Desires?

Nothing, answered I, was a greater <206> Curiosity to Astronomers, or more glorious for the Newtonian System, than the Conjunction of Jupiter and Saturn which happened in the Beginning of the present Age, an Age so fertile in the most surprizing Events. These two great Planets were to approach each other, which the vast Extent of their Orbits, and the Time employed in describing those Orbits, had not very often suffered them to do.

If it could ever by hoped to see the Effects of this mutual Attraction in the Disturbance and Alterations in the Motions of the Planets, it was upon this Occasion when the two most powerful in the whole Solar System approached each other, at a Distance however of three hundred and fifty Million of Miles. This was an Observation in great, as decisive for the Newtonian celestial System as the Experiments of refracting the coloured Rays with a second Prism, in order to prove whether Colour was a Modification of Light or no, was in little. The Curiosity in this Affair was so much the greater, because the New <207> tonian System was then in its Infancy and Time which gives Strength to Truth, and in which Error vanishes away, had not yet decided any thing to the World in its Favour.

The Disturbance which Jupiter the greatest of all the Planets occasioned in the Motions of Saturn, and that which this Planet reciprocally exercised upon the Satellites of Jupiter, were so considerable that they could not escape the Observations and Testimony of Astronomers, even those who were the least inclined to adopt the system, whom a Difference of Opinion from the Wagers held against it, might easily induce to regard this Phænomenon in a very superficial Manner. And Sir Isaac Newton had the Pleasure of extorting from his very Enemies so strong and solemn a Confirmation of his System. What are the Triumphs of the Cæsars and Alexanders (those miserable Conquerors who overturned two Particles of this Globe) when compared to the philosophical Triumph of him who first discovered the vast Extent of the Universe?

Astronomy, said the Marchioness, <208> has by this Triumph amply rewarded Sir Isaac Newton for defending it so well in the Affair of total Eclipses. This mutual Assistance, this Commerce, if I may use the Expression, of Truth, must certainly be a very great Honour to the Science. This Commerce, answered I, was never more evidently seen than in Attraction. We may affirm that every Science with a Sort of Emulation contributes to confirm this Truth, just as the whole World anciently did to the raising the Roman Empire.

Tho' I told you that the Effects of Attraction are more remarkable in the Heavens that any where else, yet it is also very evident in all Natural Philosophy, Hydrostatics, Chemistry, and Anatomy itself. Mr. Muscembrook (who even in Philosophy presents the Character of a true Republican) confesses with that Freedom that becomes a Member of the Belgic State, that for the Space of many Years spent in the greatest Variety of Experiments, he has observed in all Bodies certain Motions and Effects which could not be explained <209> or understood by Means of the external Pressure of any ambient Fluid: But that Nature proclaims aloud a Law infused in Bodies by which they are attracted, without a Dependance upon Impulsion. Chemical Fermentations, the Hardness of Bodies, the round Figure of Drops of Water, and of the Earth itself, the Separation of the Juices in the human Body, the Suction of Water by Spunges, its Ascent in those Tubes which from their extreme Smallness are called capillary, and a thousand other Things are incontestable Arguments for this Attraction. I believe that after so many repeated Proofs you will permit me to introduce it triumphantly into Optics, in order to explain the Effects which depend on this mutual Attraction betwixt Light and Bodies. It would be very extraordinary, said she, if I should refuse to admit the mutual Attraction of Light and Bodies, after I have seen the Sun and Saturn attract each other at so enormous a Distance.

Not to say any thing more of Diffraction, answered I, is not Refraction <210> an Effect of this attractive Power? Does it not arise from hence, that the Mediums through which the Light passes are indued with this Force in a greater or less Degree, according to the greater or less Density of the Medium? Otherwise the immense Force of the Earth which attracts every Thing to herself, would render it impossible for any Prism, if it were as big as Pike Teneriff, to attract the smallest Ray of Light.

While the Light passes through the same Medium because it is equally attracted on all Sides, it will not decline to any, but more forward according to that Direction it received from the Sun or any other luminous Body: If in its Way it meets with another Medium of a greater Force (such as Glass for Example compared to Air) must it not decline towards this Medium and immerse into it, approaching more or less to a Perpendicular as the Attraction of the new Medium is more or less powerful?

In going out of Glass into Air, the <211> Light is again attracted by the Air and the Glass; but because the Force of the Glass is greater than that of the air, the Light will remain behind the Surface of the Glass from whence it goes out, or behind that of the Air into which it enters, and which immediately touches the Glass itself.

Thus you see how easily by the Help of Attraction is explained a Phænomenon, which Des Cartes could not account for, unless by supposing that Light could with greater Facility pass through dense than rare Mediums; which is in other Words saying, that what makes the greatest Resistance to all other Bodies, must by some other Privilege of which I am ignorant, make the least Resistance to this. It is surprising to see how all that Experiments have discovered to happen in Refractions, are geometrically deduced from this Explication of the Newtonian Philosophy.

To me, replied the Marchioness, who cannot enter into the Sanctuary of the Mathematics, it seems a sufficient Proof <212> that as the attractive Force is greater in Proportion as the Density of the Medium is so, the Refraction must in that Case be proportionably greater.

The Dutch, answered I, have found it to be much greater in Nova Zembla than here among us. The Air is extremely cold, and consequently dense in this Country, the Habitation of white Bears, and some Europeans, the miserable Victims of the Avarice or Curiosity of their Species.

By the Help of this very great Refraction they had the Pleasure of seeing the Sun after his long Absence many Days before the Science of Cosmography had permitted him to appear, and the Density of the Air, which generally oppresses and damps the Spirits, served in this Retreat of Misery and Darkness, to revive their Imagination by an unexpectedly early Light.

We may at present hope for more exact Observations upon the Density of the Air and the Refractions of this Climate, which have never yet been examined by philosophical Eyes. A learned <213> Company is now setting Sail from France to the Bothnic Gulf, and another to Peru, in order by their united Observations to determine if possible the true Figure of the Earth, and animated by the Love of Science, have the Resolution to change the Gardens and the delightful Seats of Pleasure in their own Country for the frozen Rocks and Desarts of Lapland.

In North America the Colds are incomparably more sharp than in Europe at the same Distance from the Pole. There are in those Seas Mountains of Ice of the same Date perhaps as the World, among which are sometimes found Ships at full Sail as motionless as upon dry Land.

Doctor Halley, whom the English Nation regards with the highest Respect as the Friend and Companion of the great Sir Isaac Newton, and whose Thoughts are employed on no Objects that are little or trifling, believes that these Countries were once perhaps nearer the Pole than they at present are, that a Comet by giving a Shock to the Earth <214> altered their Situation and by this Means set them at a greater Distance. But that notwithstanding this the great Magazine of Ice formed before this terrible Shock still remains, nor has the Heat of following Ages ever been able to melt it. Hence proceed the sharp Colds and a stronger Refraction caused by them.

Certain English Sailors who above a Century ago endeavoured without Success, to find in North America a Passage to the Southern Sea, were obliged to spend the Winter in an Island very little more North than London. Every Thing there was transformed to Ice, the House they had built, the Sea, the Ship, and even themselves.

They were obliged to cut the most spirituous Wine with a Hatchet, and the Refraction was so strong that they observed the rising Moon of an oval Figure extremely long, and the Sun when at the Horizon to be twice as big in Breadth as in Length. The Air was sometimes so very clear in the Depth of that severe Winter, that they disco <215> vered more Stars by two Thirds than are usually seen, and the milky Way appeared evidently to the naked Eye a Collection of Stars. Thus in these Countries there would have been no Need of a Democritus to conjecture it among the Dreams of the ancient Philosophy, nor a Galileo to verify it by the Assistance of the Telescope.

From a great Number of Experiments made in England it evidently appears, that the refractive Power of the Air increases in Proportion to its Density, which is true likewise in other Mediums which refract the Light, but however with some Exceptions. Air, Water and Glass sensibly observe this Proportion. But oleaginous and sulphureous Liquors, and which consequently are combustible, have a greater Refraction than Liquors of another Nature even if they are more dense. Oyl which is less dense than Water, as appears by its swimming upon it, has however a great Force in refracting the Light.

Alas! interrupted the Marchioness, I am a great Enemy to Exceptions, and <216> I have a most mortal Aversion to the but in Conversation. Every one who takes it into his Head to rail against our Sex before our very Face, will doubtless except, with a constrained but, her who has the Misfortune to be present. Satire so agreeable to the Malignity of our Mind, becomes cold with these Exceptions, our Self-love is not sufficiently flattered, and Truth loses too much when it becomes less general.

Exceptions of this Sort, answered I, are properly only new Truths which arise from the Discovery of many Causes, which joined together generally concur to produce a certain Effect. This greater Refraction in a less Density of Medium arises from another particular Correspondence between these Liquors and Light. It acts upon them more than upon any other Sort, by agitating, warming, and inflaming them more easily. It is just then, on the other Hand, that these in Return should act more upon the Light than other Mediums do, by breaking and refracting it in a greater Degree.


May we not conclude from hence that this Force resides chiefly in the sulphureous Parts of Bodies? For this Reason boiling Water, in which these Parts are more disengaged, has a greater attractive Force than cold Water.

In general, Warmth and Rubbing augment the attractive Force which is in Bodies, or make it appear it in a particular Manner. Amber, all Sorts of pellucid Gems, Glass, Hair, and many other Things when they are rubbed, discover the Force called Electrical, and which is communicated to other Bodies, carried to incredible Distances, and whose Effects are surprizing beyond all Belief.

If a Tube of Glass be rubbed till it gets hot, it will attract light Bodies as Leaves of Gold or Cotton, and afterwards drive them at a Distance. It will raise a Sort of Tempest in a Heap of little Pieces of burnt Paper by attracting and then tumultuously repelling them from itself. In short, this Force is a Species of magic Wand, that communicates and awakens a <218> Power in Bodies, which had before lain dormant and inactive.

A Ball of Ivory fastened to a Cord of nine hundred or a thousand Foot long acquires the same Power of Attraction and Repulsion, if the electrical Tube be applied to the other End of the Cord a thousand Foot distant. You had very great Reason, replied the Marchioness, to call this Tube a Sort of magic Wand, for it really produces incomprehensible Effects. At least, it is a Mystery to me that it should with such Rapidity draw the little Bodies to itself, and afterwards with a Sort of Disdain remove and drive them away.

Observation, answered I, which has hitherto been our Guide and Clue in the intricate Labyrinth of Natural Philosophy, will continue to give us the same Assistance in that little Way we have yet to go. It has conducted us to the Discovery of new Properties in Light and Colours, which have opened a new Scene of Optics to Philosophy. It has conducted us to the Discover of Attraction in the most secret Retreats <219> of Bodies; which is likewise a new and surprizing Quality of Matter, by which all Natural Philosophy is changed and renewed; and now this faithful Guide leads us to Repulsion, whose Effects in Nature are no less considerable and surprizing.

Are we not to impute it to this Force, that Flies are able to walk upon the Water without wetting their Legs, and that the Particles, which fly off from Bodies by Means of Heat or Fermentation, are set at so great a Distance from each other as to possess an infinitely greater Space than they did at first?

The Air after being compressed may be dilated to such a Degree as to possess a Space more than eight hundred twenty six thousand times greater than it did when compressed, and this without being heated, which would dilate it still more. That you may understand this Force has no less Influence in Heaven than Earth, the famous Comet of 1682 approached so close to the Sun that it was heated two thousand times more than a red hot Iron. The Vapours arising <220> from it, and cast at a Distance from each other by the repulsive Force, adorned it with so formidable a Tail that it took up in Heaven the Length of eighty Million English Miles. It would have been very bad for us to have been near, and involved in it, for instead of gaining a Ring or another Moon, we should have been calcined and burnt to Ashes like a little Stone in the Focus of a burning Glass.

Some Persons so taken up with the Phantoms of the uncertain Future, as not to see the Fugitive Present, expect that one Day some Comet will cause the universal Conflagration of this Globe. Comets have perhaps anciently produced a Deluge, have struck against the Earth and overset every Thing in it, and who knows but one Time or other they may bring a Conflagration upon it, after which laying aside its old Spoils like the Snake, it may again grow young and be renewed, and our great Theatre change both its Scenes and Actors.

The present, replied she, is so various and agreeable as it now stands, that <221> I am greatly deceived if it may not divert us as yet a long Time without any Change.

But perhaps it is to these Comets that we are obliged for one of the finest Changes, and what we every Day enjoy. They are perhaps the ingenious Mechanists that have rendered our Theatre capable of being turned round like that of Curio, so famous in Antiquity, where the Roman People, the conqueror of the World, the Race of Heroes, and the Portion granted to Mankind by the immortal Gods, sat upon a frail Machine, in which they could not even applaud their public Shews without Danger. We at present perhaps owe to some Comet (without apprehending any calamitous Accident) the Rotation of our Earth, the perpetual and constant Succession of Light and Shadow; in short, the pleasing Variety of Day and Night. One of these perhaps by giving us a Shock has occasioned this Motion in us as well as the other Planets which are discovered to be indued with it.


Before this we had, like the cold Inhabitants of the Pole, six Months Day and six Months Night, but without gaining like them either a strong Refraction or a long Twilight to anticipate and prolong our Day. A little Moon-light would from Time to Time have faintly illuminated that long and melancholy Darkness. What Optics, what Colours could we ever have had for six continued Months, unless the Comet with its Shock had lent us its Assistances. Since every Thing, replied the Marchioness, stands well at present, Heaven guard us for the future from the Proximity of any one of them, from the Shocks, the Conflagrations and the Deluge with which they menace us, and from that repulsive Force that renders them so very formidable. But are not these the Riddles as well as Terrors of Natural Philosophy, that the same Bodies should be both attracted and repelled?

I do not know, answered I, after some Pause, whether I ought to introduce you any farther into the Sanctuary of the Newtonian Philosophy. There are <223> in this certain Mysteries still deeper and more sublime than those to which you have already been admitted. You should now invoke those Spirits the first-born Sons of Light, the Guardians of those secret Truths which they imparted to our Philosopher, that they would suffer me to discover to you Things concealed from the Sight of Mortals, and deeply immersed in a gloomy Mist and the most profound Night. You must now entirely divert yourself of those few Remains of Profaneness which may still attend you. Tell me, Madam, what Courage do you feel for Truth? The same, answered she, that a brave Soldier feels to follow his Captain wherever Valour calls. I follow you without Fear wherever Truth leads the Way.

You appear, answered I, to think it a Riddle in Philosophy that the same Bodies should be both attracted and repelled; and indeed there is some Reason for your thinking so. But would not the Riddle be still greater, if I tell you that these two contrary Forces, the <224> attractive and repulsive, are of the same Nature, and in short it is only the same Force which discovers itself in different Manners and various Circumstances? The Marchioness at this could scarce forbear laughing. What! said she, do you call the attractive and repulsive Force the same Thing? One acts directly contrary to the other, since the first attracts and the last repels. Are these those sublime Mysteries in Natural Philosophy of which you hardly thought me worthy, and which you have introduced with so much Apparatus? Does not all this greatly resemble the Physician in Moliere, who affirmed roast and boil'd to be the same Thing?

What! answered I, do you ridicule the most sacred Things in Natural Philosophy, and of whose Use you are yet ignorant? What a Spirit of Prophaneness still remains in you! But you shall quickly be punished. Remember the Conclusion which you yourself just now deduced from that very Attraction you made so great a Difficulty to admit. Ladies of all Persons have least Reason <225> to be surprized that the same Thing should produce contrary Effects. Do not the highest Reserve and an evident Partiality often proceed from the same Principle, and teach Connoisseurs to draw the same Conclusion from them? The Sun hardens and softens according to the different Circumstances in which he exercises his Heat.

This Truth is no less evident in the most noisy Affairs of human Life than in the Phænomena of Gallantry and Natural Philosophy The same Thirst of leaving an empty Name and Living in the imaginary Breath of Posterity, burnt the Temple at Ephesus in Asia, and precipitated a Roman and his Horse into an open Gulf in the Midst of the Forum at Rome. The same Passion made Curtius an Hero, and Erostratus an Incendiary.

There are certain Things which to the Vulgar, nay even the philosophical Vulgar, may appear as evident Contradictions in the same Man, whom some upon that Account have imagined to be double (as others did the Governor <226> of the Universe) so then what the one willed the other disapproved; but are not these Contradictions the necessary Consequences of the same Passion and the same Motives?

Thus the very same Cause, by which Bodies are attracted, may in some Circumstances repel them. There are found certain Analogies between these two Forces, which give us great Reason to conclude that they are in reality only the same Force which produces different Effects.

It is generally found that where the attractive Force is weak, the repulsive too is weak, and where the one is strong, the other is strong also. Refraction which depends on one of these two Forces, and Reflection on the other, are both effected where there is a Surface that separates two Bodies of different Density. For while the Rays proceed in the same Medium and do not meet with one of a different Density, they are neither reflected nor refracted.

Those Rays which are most refran <227> gible, are likewise more easily reflected than the rest. In Bodies by which the Light is more refracted it is likewise more strongly reflected. And in general, where the attractive Force is greatest, the reflective and repulsive is greatest also. Diamonds, which refract the light very strongly, give it in Proportion a stronger Reflection. Hence proceed the Vivacity of their Colours and their fine sparkling Lustre.

These Analogies, replied the Marchioness, are very pretty and very good, and so are the Examples by which you introduced them, and very strongly reproach my Rashness. I am heartily penitent for my Fault in laughing when I should have admired, and despising what I ought to have regarded with Veneration. But did not you tell me that Reflection happens when Light meets with the solid Parts of Bodies and is from thence repelled? This Explication seemed very intelligible to me, and perhaps more so than what you at present intimate.

It is Des Cartes, Madam, who gave <228> you this Explication, and not I, therefore you may be in some Apprehensions about it. An ingenious Author gives a very useful Piece of Advice, that in Philosophy we should be as diffident of what we imagine ourselves to understand very easily, as of what we do not understand at all.

If Reflection was made when the Light meets with the solid Parts of Bodies (as you so clearly understand it ought to be) do you know what an Inconvenience would from thence arise in Nature, you would no longer have either Toilette or Looking-Glasses?

A Surface, however smooth and polished it may be, is however full of Protuberances and sensible Irregularities discoverable by a Microscope. Imagine all Bodies which you believe the most smooth and polished, to be like Water when it is rufled by the Wind. The Light must be reflected from all Bodies irregularly as it is from Water thus rufled, and could never be sent back with that Regularity as is necessary for you to see yourself in <229> a Looking-Glass. Is not this paying a great Price for your fine Explication? Is it really true, answered she, that it will cost so dear? Perhaps you put me into a greater Terror than the Danger deserves. Is it not possible that these Irregularities in the Surfaces of Glasses, tho' sensible to the Microscope, may yet be insensible to the Light?

You are grown extremely difficult, Madam, answered I. The Protuberances and Cavities in the most smooth and polished Glasses are when compared to a Particle of Light, what the Alps of Pyrenees would be in respect of a Tennis-Ball. The Irregularities of a Looking-Glass are discoverable by common Microscopes, but there is no Microscope so perfect as to shew the Pores of Diamond, through which the Light however passes very copiously. It would be a terrible Circumstance for us, if the Particles of Light were not almost infinitely small. The Force of Bodies is reckoned from the Quantity of Matter that they contain, which is called the Mass, and from their Velocity; the greater <230> therefore their Mass and Velocity, the greater is their Force.

The Particles of Light have an incredible Velocity, for they come from the Sun to the Earth in about eight Minutes, and thus in eight Minutes they run through a Space of eighty one million Miles. Their Velocity then being so extremely great as to exceed more than ten million times that of the swiftest English Horses, their Mass must be almost infinitely small, or else a single Particle of Light, instead of animating and reviving all Nature by its Appearance, would produce upon our Earth the most terrible Effects of a Cannon.

The good Effects, answered the Marchioness, of that Diffidence we ought to shew to Men, extend likewise to Philosophers; for by this Means the one gives greater Proofs of what we desire should be true, and the other of what really is so. I will for the future be very careful not to believe you too hastily.

For this time at least, answered I, you cannot accuse yourself of too much <231> Credulity. It will never burthen your Conscience that you have not had sufficient Arguments to believe that Reflection is not made by the meeting of Light with the solid Parts of Bodies. For besides the great Inconvenience which would arise if this Supposition were true, Observation informs us, that Light, transmitted through a Piece of Glass, suffers a stronger Reflection at going out of the Glass than it did at entering into it. Now how is it possible that Light should find more solid Parts in the Air than in the Glass itself, to occasion this stronger Reflection?

Besides, if Water or Oyl be placed immediately behind the Glass, the Reflection becomes weaker. Will the Light find fewer solid Parts in Water or Oyl than in Air?

Lastly, if the Air which is behind the Glass be taken away by an Instrument made for that Purpose, the Reflection will be much stronger than it was before the Air was removed. Will you say that Light meets with a greater Number of solid Parts in a Vacuum than in <232> the Air? Heaven forbid that I should say it, replied the Marchioness; I will rather make the repulsive Force to be the Cause of Reflection. In these Cases, answered I, it is not the repulsive but the attractive Force. When a Ray goes out of the Glass into Air, it is attracted by the Air and the Glass; hence that Part of it, which was nearest the Glass, returns back as if it had been reflected, if the Air be entirely taken away, this Part being extremely attracted by the Glass, and hardly any Thing by what remains after the Air is removed, it returns back again almost entire.

But if Water or Oyl (which attract the Ray much more strongly than the Air does) be placed behind the Glass, a less Part of the Ray will return back than there did while the Air remained. Lastly, when the Forces of the two Mediums are balanced, as for Instance, when a Liquor be applied to the Glass of pretty near the same Density, or another Piece of Glass, the Ray must pass entire, and there will be no Reflection.

In general, we may affirm that the <233> attractive Force is the Cause of the Reflection of the Rays when the Light, passes from a dense into a rare Medium; and the repulsive, when it passes from a rare to a dense.

In both Cases, since the attractive and repulsive Force are propagated at some Distance from Bodies, the Light is reflected notwithstanding its Distance from the reflecting Body. Just as when it begins to be refracted, it is somewhat distant from the refracting Medium, in the same Manner as it is from the Extremity of Bodies when passing near them, it is turned out of its direct Path and curved by Diffraction. Hence you see that solid Parts and Des Cartes Explication have less to do with Reflection than ever.

Poor Des Cartes, continued she, is attacked in his very last Retrenchments. There is nothing wanting to compleat his Overthrow but for us to assert, that as Light is not reflected from solid Parts, so neither is it transmitted from the Pores of Bodies, and thus he may return back like that momentaneous Alexander <234> of the North, who after the most rapid and noisy Victories, at length lost the best of his own Dominions.

In order to deny him every Thing and give him Leave to return home as soon as he pleases, we will at least deny that the Quantity or Magnitude of the Pores in Bodies, contributes any Thing to their Transparency. It is proved on the contrary, that if the Pores of a Body, for Instance Paper, be filled with Water or Oyl, it loses its Opacity and becomes transparent; whereas if the Pores in a Body be multiplied, as when Glass is reduced to Powder, it loses its Transparency and becomes opaque.

It is in Homogeneity we are to seek the Cause of Transparency. If there be many Pores in a Body, and these be filled with a Matter different from that of the Body itself, the Light will meet with a thousand Reflections and Refractions in the internal Parts, so and thus it will be utterly extinguished. The Air ceases to be transparent when it is cloudy, tho' it is then lighter than when it is <235> clear, and consequently more porous. Its Opacity can arise from no other Cause but that it is at that Time heterogenous, which makes the Rays that pass through it suffer innumerable Reflections and Refractions, by which Means they are very soon stifled and extinguished. Thus the Froth of Champagne is opaque, tho' much more porous and lighter than the Wine itself.

This seems to furnish us with an Argument to infer, that the Heavens cannot be filled with Matter however rare it be supposed, even if all contained within the vast Orb of Saturn, and tho' its Pores should be as small as you can possibly conceive, might, after being perfectly united so as to leave no empty Space, by grasped in one Hand.

What strange Relation is this you tell me? replied the Marchioness. Is the Newtonian Philosophy some golden Fleece which we must not undertake the Conquest of, till we have first passed through a thousand strange Portents, and subdued a thousand Monsters of Imagination.


Do you believe, answered I, that Gold the precious Substance for which Mankind do and suffer so much, and for which we feel the greatest Thirst when we ought to be most satisfied, Gold I say, and Diamonds themselves, the most shining Work of Nature, notwithstanding their great Weight and Gravity, contain a great Quantity of Matter? It will appear strange when I tell you how very little that Quantity really is when compared to that Vacuum between the Parts, and which to our deluded Eye seems perfectly full.

The solid Parts contained in a Piece of Glass are no more considered with regard to its Extent, than a Grain of Sand to the terraqueous Globe. It is very surprizing how little solid Matter there is in the World, and with how few Materials, if I may use that Expression, it is built. Perhaps if you knew the Truth you would be afraid you walked upon Cotton, and were in Danger of crushing it under your Feet, even if they were as light as those of the swift Camilla.


Now if the Matter of the Heavens be imagined unconceivably rare and subtle, yet the Light which, notwithstanding its prodigious Velocity, employs according to the last Calculations six Years in coming from the Starts to us, must be intirely extinguished by that Multiplicity of Reflections and Refractions which it must suffer in that immense Passage; just as a numerous and flourishing Army in a long March must decrease and perish by the Fatigues and Obstacles it meets upon the Road.

I see with Pleasure, said the Marchioness, how the Properties of our Light conduct us to empty Heaven, and after having set the Earth in Motion, to clear its Way. The Diffractions too, answered I, which the Light would suffer from the Particles of this Celestial Matter, would contribute not a little to extinguish it, in the same Manner as they would do in Bodies that are very porous and heterogeneous. It is surprizing that in the Notes which Perrault wrote upon Vitruvius, he seems, if I remember right, to have had some faint Notion <238> of this Truth. Rarefaction, said he, (that is, a Distance of Parts) renders Bodies opaque, because these, which were at first homogeneal when rarefied, become heterogeneous.

It seems much more surprizing, said the Marchioness, that any Person could clearly see and demonstrate that two Things so very opposite as Reflection and Refraction should yet arise from the same Cause. This I must confess will always be a Wonder to me.

The Facility, answered I, and the Obstacles which the Light meets in passing out of one Medium into another, are almost in the same Case. Perhaps an exceedingly subtile Fluid diffused int eh Confines of the Mediums extremely quick in its Vibrations, and in which the Light by Percussion excites an undulating and tremulous Motion (as a Stone does in Water, or a Voice in the Air) is the Cause of both the Facility and Obstacles in Question. Thus if the Light happens to be in the Concavity of the Waves of this Fluid, it passes freely through it, but if in <239> their Summit, it is drove back again.

Hence come the Fits of easy Transmission and Refraction, that is, the same Ray of Light is in one Moment transmitted, and in the next reflected; and because the Vibrations of this fluid are exceedingly rapid, the Rays appear to us both transmitted and reflected at the same Time.

But we are now arrived to the Confines of Nature where our Ideas grow dark and confused; these are the Barriers of Knowledge, beyond which no Force of human Faculties is permitted to proceed, I myself perhaps have run too great a Length.

Sir Isaac Newton under the Form of Questions proposed many Things which are probably the Recesses where Nature withdraws to conceal herself from mortal Eyes. The Analogy betwixt Sound and Colour, the strange Metamorphoses of Light into Bodies, and Bodies into Light, the two-fold and surprizing Refractions of Island Chrystal, Rock Chrystal, and that Sort lately discovered in Brasil, will always be impenetrable <240> Enigmas to Mankind, since this OEdipus was not able to solve them.

How very different from the modest Doubts of this Legislator of the Wise are the rash Assertions of the Seducers of the Multitude. These however promise those very Men, whom they have always deceived with the same Flatteries, to open the Temple of Truth so often tried in vain, in the most expeditious and easy Manner by the Help of certain new Principles; just as others with certain new Systems of their own spread artful Nets for human Avarice, and promise at once to enrich a Nation which they have always impoverished by the very same Schemes. The Pleasing and vain Delusion of Hope leads some in Haste to the Bank, others to the Academy. The Beginning of an Affair is generally agreeable to our flattering Expectations. The Wind is commonly favourable to the Ship just loosed from the Port, and two bright Eyes give an agreeable Invitation at their first Appearance. The Bank by converting Hopes at first into Gold pre <241> serves and increases its Reputation, and Philosophy by judicious Prefaces maintains its Honour, more successful in banishing old Errors, than substituting new Truths in their Place. Thus they who by a prudent Diffidence soon extricated themselves from the Snare, either brought back a reputable Increase of their Fortune, or were rationally delivered from their past Prejudices. But there are very few so wise as not to lose the present in forming Projects for the future, or not to make the Happiness of today, a Step to the Misery of to-morrow. The Cheat at Length is discovered, and the first are left with their Scrutoirs, full of Notes worth nothing, while the last have their Heads embarrassed with Notions of Pressure, Rotations, Globules, and Vortices, the false Coin of Philosophy. Sir Isaac Newton, guided by a slow yet sure Experience, promises no more than what Experience is able to perform. Where that leaves him he stops, by its Assistance he distinguishes Truth from Falshood, Evidence from Probability, and in the Extent of his <242> own, discoveries the Limits of the human Understanding.

Are not the Rays of Light (says Sir Isaac Newton) very small Bodies of different Sizes, the least of which make Violet, the weakest and darkest of all the Colours, and more easily diverted by the attractive Force of the Prism from the right Path? And the rest, as they are bigger and bigger, make the stronger and more lucid Colours, Blue, Green, Yellow and Red, and are more difficultly refracted, in Proportion to the greater Strength of the Colours, and the larger Size of the Bodies that compose them?

It is certain that the Rays of Light differ from each other in Colour, Refrangibility, and the Force with which they strike our Senses. Scarlet dazzles the Sight, the Azure of Heaven languidly moves it, and the Verdure of a Meadow strikes it with a very pleasing Sensation.

Only one of these Differences, said the Marchioness, had been sufficient to make an ordinary Philosophy put an absolute Difference in the Size of the Particles of <243> Light, whereas two or three are hardly enough to supply ours with a Conjecture.

In the vast and unlimited Perspective of Nature (answered I) there are Objects that we are for ever condemned to see languid and confused, without hoping that any Telescope can make them appear either less distant or more distinct. The Moderation of our Philosopher, in never affirming any Thing to be true which was not demonstrated by Observation, may serve for an Example to the most rash Asserters. Who could have more Reason to think himself capable of ascending Heaven, or bringing the Secrets of Nature in Triumph from thence, than Sir Isaac Newton, who, poised upon the Wings of Geometry, could take his Flight through immense Spaces, till then impenetrable to human Curiosity?

What a strange Condition, replied the Marchioness, is ours! We know what Size in a Particle at a great Distance from our Sight is necessary to reflect a certain Colour, but what is this Colour itself which we have always immediately <244> before our Eyes? Hardly can we form any Idea of it by a feeble Conjecture: In one Case we have the Sight of a Lynx, in the other we are absolutely blind. There our Senses are refined beyond what we could have expected; here they seem to abandon us all at once.

There are some Persons, answered I, who have believed that the many Difficulties which attend our little Degree of Knowledge, the great Number of Systems, the various Emblems of human Ignorance, and that continual tantalizing which Philosophers suffer in their Searches after Truth, proceed from no other Cause than our want of a sixth natural Sense which might reveal a great Part of what is at present hid from us, and escapes perhaps those five Senses given us by Nature to lay hold on external Objects, and bring them to the Mind.

As there are certain Animals among us, who, by virtue of Senses, of which we perhaps have no Knowledge, foresee the Change of the Seasons, and approach of the Morning, and, without having <245> read Dioscorides or any other Botanist, can amidst a thousand others distinguish that salutary Herb that cures their Hurts, who knows but in some other System, (in the World of Jupiter perhaps,) there may be Animals which, more sharp-sighted than our Philosophers, may discover the Size of those Particles that compose the Variety of Colours, and in what manner, without the Assistance of Ropes or Pullies, they may attract Saturn at a Distance of more than three hundred and fifty Millions of Miles?

But in return, as in that Planet, which is not depopulated by the Race of War, the Inhabitants have no Idea of the Pleasures of Love, so, according to the agreeable Historian of these Worlds, every Thing is diversified, and put in a just Balance; those People who are throughly acquainted with the Nature of Colours, may perhaps want the Sense proper to give them a Perception of the most agreeable Harmony of those Colours upon the Cheeks of their Chloes, and tho' perfectly skilled in the Attractions of the Planets, are perhaps <246> insensible to the more pleasing Attractions of Beauty, which are preferable to any Speculation whatever.

But however it be with regard to our present Speculation, the most vain perhaps of all others, it is not for our Advantage to seek Occasions to put us in Mind of our Defects, nor be so ingenious in tormenting ourselves. We shall not be destitute of either Knowledge or Pleasure, if we make a good Use of the Senses fallen to our Lot. Though you know nothing wherein consists the Nature of Light and Colours but by Conjecture, there will not perhaps be wanting some Persons to affirm you know much more of it than is proper for a Lady. The Fault will be thrown entirely upon Me, who upon those few Verses which gave Occasion to our Subject, have made you a Comment, long enough for a Poem upon the Newtonian Philosophy. It will be well for it, if you dissemble your Knowledge with those Persons who ridicule what they ought to learn, and if to the Science of Natural Philosophy you join that of the World.


What, cried the Marchioness, am I so learned that I ought to study to be ignorant? May I venture seriously to call myself a Newtonian? You have already renounced your philosophical Errors, answered I. The Light of Newtonianism has dissipated the Cartesian Phantoms which deluded your Sight. You are now really a Newtonian, and it is no small Advantage to Truth that you are so. I will some time give a History of the fine Conquest I have acquired for Her, and I am certain if I could give a Just Description of my fair Disciple, my Book would never want Readers, nor true Philosophy a numerous Train of Proselytes. You shall be the Venus that must lend the agreeable Cestus to this austere Juno, and give her those attractive Charms that will render her engaging and amiable to Mankind.

F I N I S.

[1] Mr. Swinden in his Treatise of the Nature and Place of Hall.

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