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Of Matter and Motion.
By Matter, my dear friends, we mean the substance of all things, or that of which all bodies are composed, in whatever form or manner they may present themselves to our senses: for this top, Tom Wilson's ivory ball, the hill before us, that orange on the table, and all things you see, are made of matter differently formed.
When a body is in motion, as much force is required to make it rest, as was required, while it was at rest, to put it in motion. Thus, suppose a boy strikes a trap-ball with one hand, and another stands close by to catch it with one of his hands, it will require as much strength or force to stop that ball, or put it in a state of rest, as the other gave to put it in motion; allowing for the distance the two boys stand apart.
No body or part of matter can give itself either motion or rest: and therefore a body at rest will remain so for ever, unless it be put in motion by some external cause; and a body in motion will move for ever, unless some external cause stops it.
This seemed so absurd to Master Wilson, that he burst into a loud laugh. What, says he, shall any body tell me that my hoop or my top will run for ever, when I know by daily experience that they drop of themselves, without being touched by any body? At which our little Philosopher was angry, and having commanded silence, Don't expose your ignorance, Tom Wilson, for the sake of a laugh, says he: if you intend to go through my Course of Philosophy, and to make yourself acquainted with the nature of things, you must prepare to hear what is more extraordinary than this. When you say that nothing touched the top or the hoop, you forget their friction or rubbing against the ground they run upon, and the resistance they meet with from the air in their course, which is very considerable though it has escaped your notice. Somewhat too might be said on the gravity and attraction between the top or the hoop, and the earth; but that you are not yet able to comprehend, and therefore we shall proceed in our Lecture.
A body in motion will always move on in a strait line, unless it be turned out of it by some external cause. Thus we see that a marble shot upon the ice, if the surface be very smooth, will continue its motion in a strait line, till it is put to rest by the friction of the ice and air, and the force of attraction and gravitation.
The swiftness of motion is measured by distance of place, and the length of time in which it is performed. Thus if a cricket-ball and a fives-ball move each of them twenty yards in the same time, their motions are equally swift; but if the fives-ball moves two yards while the cricket-ball is moving one, then is the motion of the fives-ball twice as swift as the other.
But the quantity of motion is measured by the swiftness of motion, as above described, and the quantity of matter moved considered together. For instance, if the cricket-ball be equal in bulk and weight to the fives-ball, and move as swift, then it hath an equal quantity of motion. But if the cricket-ball be twice as big and as heavy as the fives-ball, and yet moves equally swift, it hath double the quantity of motion; and so in proportion.
All bodies have a natural tendency, attractions, or gravitation towards each other. Here Tom Wilson, again laughing, told the Company that Philosophy was made up of nothing but hard words. That is because you have not sense enough to enquire into, and retain the signification of words, says our Philosopher. All words, continued he, are difficult till they are explained; and when that is done, we shall find that gravity or gravitation will be as easily understood as praise or commendation, and attraction as easily as correction; which you deserve, Tom Wilson, for your impertinence.<9>
Gravity, my dear friends, is that universal disposition of matter which inclines or carries the lesser part towards the centre of the greater part; which is called weight or gravitation in the lesser body, but attraction in the greater, because it draws, as it were, the lesser body to it. Thus all bodies on or near the earth's surface have a tendency, or seeming inclination, to descend towards it middle or centre; and but for this principle in nature, the earth (considering its form and situation in the universe) could not subsist as it is: for we all suppose the earth to be nearly round, (nay we are sure it is so, for my Lord Anson and many other gentlemen, you know, have sailed around it) and as it is suspended in such a mighty void or space and always in motion, what should hinder the stones, water, and other parts of matter falling from the surface, but the almighty arm of God, or this principle or universal law of nature, of attraction and gravitation, which he has established to keep the universe in order. To illustrate and explain what I have said, let us suppose the following figure to be the earth and seas. Let Tom Wilson stand at this point of the globe or earth where we are, and Hal Thompson at the opposite part of the earth, with his feet (as they must be) towards us: if Tom drop an orange out of his hand, it will fall down towards Hal; and if Hal drop an orange, it will fall seemingly upwards (if I may so express myself) towards Tom: and if these oranges had weight and power sufficient to displace the other particles of matter of which the earth is composed, so as to make way to the centre, they would there unite together and remain fixed; and they would then lose their power of gravitation, as being at the center of gravity and unable to fall, and only retain in themselves the power of attraction.
This occasioned a general laugh; and Tom Wilson starting up, asked how Master Thompson was to stand with his feet upwards, as here represented, without having anything to support his head? Have patience, says the little Philosopher, and I will tell you; but pray behave with good manners, Master Wilson, and don't laugh at every thing you cannot comprehend. This difficulty is solved, and all the seeming confusion which you apprehend of bodies flying off from each other is removed, by means of this attraction and gravitation. Ask any of the sailors who have been around the world, and they will tell you that the people on the part of the globe over-against us do not walk upon their heads, though the earth is round; and though their heels are opposite ours, they are in no more danger of falling into the mighty space beneath them, than we are of falling (or rather rising I must call it here) up to the moon or the stars.
But besides this general law of attraction and gravitation, which affects all bodies equally and universally, there are particular bodies that attract and repel each other, as may be seen by the Magnet or Loadstone, which has not only the property of directing the nedle of the mariner's compass when touched with it to the north, but also of attracting or bringing iron to it with one end, and repelling or forcing it away with the other end: and Sam Jones's knife, which was whet on a Loadstone some years ago, still retains the power you see of picking up needles and small pieces of iron.
Glass, Sealing-Wax, Amber, and Precious Stones, when chafed or rubbed till they are warm, will likewise both attract and repel feathers, hairs, straw &c. which is sufficient to prove that each of these bodies has a sphere of arractiuopm assined it, beyond which it will repel the same body it would otherwise attract.
When bodies are so attracted by each other as to be united or brought into close contact, they then adhere or cohere together so as not to be easily separated, and this is called in Philosophy the Power of Cohesion, and is undoubtedly that principle which binds large bodies together; for all large bodies are made up of atoms or particles inconceivably small. And this cohesion will always be proportioned to the number of particles or quantity of the surface of bodies that come into contact or touch each other: for those bodies that are of spherical form will not adhere so strongly as those that are flat or square; because they can only touch each other at a certain point; and this is the reason why the particles of water and quicksilver, which are globular or round, are so easily separated with a touch, while those of metals and some other bodies are not to be parted but with great force. To give us a familiar instance of this cohesion of matter, our Philosopher took two leaden balls, and filing a part off each so that the two flat parts might come into close contact, he gently pressed them together and they united so firmly that it required considerable force to get them asunder.
One thing I must tell you of magnetism, which seems pretty extraordinary. Master Brown took his uncle's sword, and supported it with the point downwards, by resting the shell of the hilt on the top of his two forefingers; and Master Smith was placed with his father's amber-headed cane at about three or four feet distance, where he kept rubbing the amber head round on his waistcoat. After some little time the sword began to move, tho' at that distance; and some time after that it turned quite round; but was soon turned back again by master 's rubbing the amber head backwards, or in a contrary direction
But what seems most worthy of our admiration is the Electrical Fire, which so plentifully abounds in the universe, and which is excited, or made visible, by the friction or rubbing of a glass globe. This fire, by a very simple machine, may be conveyed into the human body, every part of which it pervades in an instant, and is said to have been very serviceable in the cure of some disorders. It may be drawn from the ladies eyes, yet it leaves them no less brilliant than they were before. It may be drawn from under thunder-clouds, and is probably the same species of fire with the lightening; for Professor Richmann of Hamburgh>, who fixed a machine to bring it down from the clouds in large quantities, was killed by the stroke it gave him.
The same force aplied to two different bodies will always produce the same quantity of motion in each of them. To prove this, we put Master Jones into a boat, which (including his own weight) weighed ten hundred, on the Thames by the Mill-bank; and on the Lambeth side, just opposite, we placed another boat (of one hundred weight) with a string tied to it. This string Master Jones pulled in the other boat; and we observed that as the boats approached each other, the small boat moved ten feet for every foot the other moved: which proves what we have before observed as to the quantity of motion.
Attraction is the stronger, the nearer the attracting bodies are to each other; and in different distances of the same bodies it decreases as the squares of the distances between the centres of those bodies increase. For if two bodies at a given distance attract each other with a certain force, at half the distance they will attract each other with four times that force. But this I shall further explain in my next course of Lectures.
Two bodies at a distance will put each other into motion by the force of Attraction or Gravitation. This we know to be true by experience, though we cannot account for it; and therefore it is to be received as a principle in Natural Philosophy.<16>