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The COPERNICAN SYSTEM demonstrated to
and mo- 99. tion.
ATTER is of itself inactive, and indifferent to motion or rest. A body at rest can never put itself in motion; a body in motion can never stop or move slower of itself. Hence, when we see a body in motion, we conclude that some other substance must have given it that motion; when we see a body fall from motion to rest we conclude that has some other body or cause stopt it.
100. All motion is naturally rectilineal. A bullet thrown by the hand, or discharged from a cannon. would continue to move in the same direction it received at first, if no other power diverted its course. Therefore, when we see a body moving in a curve of whatever kind, we conclude it must be acted upon by two powers at least one to put it in motion, and another drawing it off from the rectilineal course which it would otherwise have continued to move in.
101. The power by which bodies fall toward the Earth, is called gravity or attraction. By this power in the Earth it is, that all bodies on whatever side, fall in lines perpendicular to its surface. On opposite parts of the Earth, bodies fall in opposite directions;-all toward the centre, where the whole force of gravity is, as it were, accumulated. By this power constantly acting on bodies near the Earth, they are kept from leaving it altogether; and those on its surface are kept there on all sides, so that they cannot fall from it. Bodies thrown with any obliquity are drawn, by this power, from a straight line into a curve, until they fall to the ground: the greater the force by which they are thrown, the greater is the distance they are carried before they fall. If we suppose a body carried se.
veral miles above the Earth, and there projected in a horizontal direction, with so great a velocity, that it would move more than a semidiameter of the Earth in the time it would take to fall to the Earth by gravity; in that case, if there were no resisting medium in the way, the body would not fall to the Earth at all, but continue to circulate round the Earth, keeping always the same path, and returning to the point from whence it was projected, with the same velocity as at first.
102. We find that the Moon moves round the Earth Projectile in an orbit nearly circular. The Moon therefore monstramust be acted on by two powers or forces; one, ble. which would cause her to move in a right line; another, bending her motion from that line into a curve. This attractive power must be seated in the Earth; for there is no other body within the Moon's orbit to draw her. The attractive power of the Earth therefore extends to the Moon; and, in combination with her projectile force, causes her to move round the Earth, in the same manner as the circulating body above supposed.
103. The moons of Jupiter and Saturn are ob- The Sun served to move round their primary planets: there- ets attract fore there is an attractive power in these planets. each All the planets move round the Sun, and respect it other. for their centre of motion: therefore the Sun must be endowed with an attracting power, as well as the Earth and planets. The like may be proved of the comets. So that all the bodies or matter of the solar system, are possessed of this power; and so perhaps is all matter universally.
104. As the Sun attracts the planets with their satellites, and the Earth the Moon; so the planets and satellites re-attract the Sun, and the Moon the Earth; action and re-action being always equal. This is also confirmed by observation; for the Moon raises tides in the ocean, and the satellites and planets disturb one another's motions.
105. Every particle of matter being possessed of an attracting power, the effect of the whole must be in proportion to the number of attracting particles: that is, to the quantity of matter in the body. This is demonstrated from experiments on pendulums: for, when they are of equal lengths, whatever their weights be, they always vibrate in equal times. Now, if one be double the weight of another, the force of gravity or attraction must be double to make it oscillate with the same celerity; if one have thrice the weight or quantity of matter of another, it requires thrice the force of gravity to make it move with the same celerity. Hence it is certain, that the power of gravity is always proportional to the quantity of matter in bodies, whatever may be their magnitudes or figures.
106. Gravity also, like all other virtues or emanations, either drawing or impelling a body toward the centre, decreases as the square of the distance increases: that is, a body at twice the distance attracts another with only a fourth part of the force; at four times the distance, with a sixteenth part of the force, &c. This too is confirmed from observation, by comparing the distance which the Moon falls in a minute from a right line touching her orbit, with the space which bodies near the Earth fall in the same time and also by comparing the forces which retain Jupiter's moons in their orbits: as will be more fully explained in the seventh chapter.
Gravita- 107. The mutual attraction of bodies may be projection exemplified by a boat and a ship on the water, exempli tied together by a rope. Let a man either in the
ship or boat pull the rope (it is the same in effect at which end he pulls, for the rope will be equally stretched throughout) the ship and boat will be drawn toward one another; but with this difference, that the boat will move as much faster than the ship, as the ship is heavier than the boat. Suppose the boat as heavy as the ship, and they will draw one
another equally, (setting aside the greater resistance of the water on the larger body) and meet in the middle of the first distance between them. If the ship be a thousand or ten thousand times heavier than the boat, the boat will be drawn a thousand or ten thousand times faster than the ship; and meet proportionably nearer the place from which the ship set out. Now, while one man pulls the rope, endeavouring to bring the ship and boat together, let another man in the boat, endeavour to row it off sideways, or at right angles to the rope; and the former, instead of being able to draw the boat to the ship, will find it enough for him to keep the boat from going further off; while the latter endeavouring to row off the boat in a straight line, will, by means of the other's pulling it toward the ship, row the boat round the ship at the rope's length from her. Here the power employed to draw the ship and boat to one another represents the mutual attraction of the Sun and planets by which the planets would fall freely toward the Sun with a quick motion; and would also in falling attract the Sun toward them. And the power employed to row off the boat, represents the projectile force impressed on the planets, at right angles, or nearly so, to the Sun's attraction; by which means the planets move round the Sun, and are kept from falling to it. On the other hand, if it be attempted to make a heavy ship go round a light boat, they will meet sooner than the ship can get round; or the ship will drag the boat after it.
108. Let the above principles be applied to the Sun and Earth; and they will evince, beyond a possibility of doubt, that the Sun, not the Earth, is the centre of the system; and that the Earth moves round the Sun as the other planets do.
For, if the Sun move about the Earth, the Earth's attractive power must draw the Sun toward it, from the line of projection, so as to bend its motion into a curve. But the Sun being at least
227 thousand times as heavy as the Earth, being so much heavier as its quantity of matter is greater, it must move 227 thousand times as slowly toward the Earth, as the Earth does toward the Sun; and consequently the Earth would fall to the Sun in a short time, if it had not a very strong projectile motion to carry it off. The Earth therefore, as well as every other planet in the system, must have a rectilineal imsurdity of pulse, to prevent its falling to the Sun. To say, supposing that gravitation retains all the other planets in their the Earth orbits, without affecting the Earth, which is placed
between the orbits of Mars and Venus, is as absurd as to suppose that six cannon bullets might be projected upward to different heights in the air; and that five of them should fall down to the ground, but the sixth, which is neither the highest nor the lowest should remain suspended in the air without falling, and the Earth move round about it.
109. There is no such thing in nature as a heavy body moving round a light one, as its centre of motion. A pebble fastened to a mill-stone, by a string, may, by an easy impulse, be made to circulate round the mill-stone; but no impulse whatever can make a mill-stone circulate round a loose pebble; for the mill-stone would go off, and carry the pebble along with it.
110. The Sun is so immensely greater and heavier than the Earth,* that if he were moved out of his place, not only the Earth, but all the other planets, if they were united into one mass, would be carried along with the Sun, as the pebble would be, with the mill-stone.
111. By considering the law of gravitation which takes place throughout the solar system, in another light, it will be evident, that the Earth moves round the Sun in a year; and not the Sun round the Earth. It has been shewn (106) that the
* As will be demonstrated in the ninth chapter.