ELEMENTS OF NATURAL PHILOSOPHY. CHAPTER I. Of Matter and Motion. MATTER is an extended solid substance; which being comprehended under distinct surfaces, makes so many particular distinct bodies. Motion is so well known by the sight and touch, that to use words to give a clear idea of it would be in vain. Matter, or body, is indifferent to motion, or rest. There is as much force required to put a body, which is in motion, at rest, as there is to set a body, which is at rest, into motion. No parcel of matter can give itself either motion or rest, and therefore a body at rest will remain so eternally, except some external cause puts it in motion; and a body in motion will move eternally, unless some external cause stops it. A body in motion will always move on in a straight line, unless it be turned out of it by some external cause; because a body can no more alter the determination of its motion, than it can begin it, alter, or stop its motion itself. The swiftness of motion is measured by distance of place, and length of time wherein it is performed. For instance, if A and B, bodies of equal or different bigness, move each of them an inch in the same time; their motions are equally swift; but if A moves two inches, in the time whilst B is moving one inch, the motion of A is twice as swift as that of B. The quantity of motion is measured by the swiftness of the motion, and the quantity of the matter moved, taken together. For instance, if A, a body equal to B, moves as swift as B; then it hath an equal quantity of motion. If A hath twice as much matter as B, and moves equally as swift, it hath double the quantity of motion; and so in proportion. It appears, as far as human observation reaches, to be a settled law of nature, that all bodies have a tendency, attraction, or gravitation towards one another. The same force, applied to two different bodies, produces always the same quantity of motion in each of them. For instance, let a boat, which with its lading is one ton, be tied at a distance to another vessel, which with its lading is twenty-six tons; if the rope that ties them together be pulled, either in the less or bigger of these vessels, the less of the two, in their approach one to another, will move twenty-six feet, while the other moves but one foot. Wherefore the quantity of matter in the earth being twenty-six times more than in the moon; the motion in the moon towards the earth, by the common force of attraction, by which they are impelled towards one another, will be twenty-six times as fast as in the earth; that is, the moon will move twenty-six miles towards the earth, for every mile the earth moves towards the moon. Hence it is, that, in this natural tendency of bodies towards one another, that in the lesser is considered as gravitation, and that in the bigger as attraction; because the motion of the lesser body (by reason of its much greater swiftness) is alone taken notice of. This attraction is the strongest, the nearer the attracting bodies are to each other; and, in different distances of the same bodies, is reciprocally in the duplicate proportion of those distances. For instance, 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; at one third of the distance, with nine times that force; and so on. Two bodies at a distance will put one ancther into motion by the force of attraction; which is inexplicable by us, though made evident to us by experience, and so to be taken as a principle in natural philosophy. Supposing then the earth the sole body in the universe, and at rest; if God should create the moon, at the same distance that it is now from the earth, the earth and the moon would presently begin to move one towards another in a straight line by this motion of attraction or gravitation. If a body, that by the attraction of another would move in a straight line towards it, receives a new motion any ways oblique to the first; it will no longer move in a straight line, according to either of those directions, but in a curve that will partake of both. And this curve will differ, according to the nature and quantity of the forces that concurred to produce it; as, for instance, in many cases it will be such a curve as ends where it began, or recurs into itself; that is, makes up a circle, or an ellipsis or oval very little differing from a circle. CHAPTER II. Of the Universe. To any one, who looks about him in the world, there are obvious several distinct masses of matter, separate from one another; some whereof have discernible motions. These are the sun, the fixed stars, the comets, and the planets, amongst which this earth, which we VOL. III. X |