SECTION FIFTH. PHYSICAL CAUSES OF THE MOTIONS OF THE PLANETS. FROM the uniform projectile motion of bodies in straight lines, and the universal power of attraction which draws them off from these lines, the curvilineal motions of all the planets arise. If a body be projected in a right line in open space, and meeting with no resistance, it would continue forever to move with the same velocity, and in the same direction. But when this projectile force is acted upon by any attracting body, with a power duly adjusted, and perpendicular to its motion, it will then be drawn from a straight line and forced to revolve round the centre of attraction in a circular form. But when the projectile force first given exceeds the attracting force, the centrifugal force, or tendency to fly off in a tangent, is arrested by the attractive power, and therefore its velocity becomes continually more and more impeded, until the attractive power has acquired a greater influence, and then its motion becomes gradually accelerated with a tendency to approach nearer to its point of attraction, and consequently the moving body forms an elliptical orbit. It is readily perceived, that, in two points in the orbit, the centripetal and centrifugal forces are equal, and that when its motion is both accelerated and retarded by the attractive power, it must of necessity pass over equal areas in the same space of time. As the planets approach nearer the sun, and recede farther from him in every revolution, there may be some difficulty in conceiving the reason, why the pow. er of gravity when it once obtains the victory over the projectile force, does not bring the planets continually nearer the sun in every revolution, till they fall upon and unite with him. Or why the projectile force when it once gets the better of gravity, does not continue to carry the planets farther from the sun, till it removes them quite out of the sphere of his attraction and go on in straight lines forever afterwards, or when the centripetal and centrifugal forces are equal, why it does not commence moving off in the form of a perferct circle. By considering the effects of these powers acting on each other according to the preceding description, the difficulty will be at once removed. A double projectile force will always balance a quadruple power of gravity, and when a planet is put in motion by projectile force, whether the velocity with which it moves be rapid or slow, it is continually resisted by the attraction of the sun, and consequently moves, slower until the power of attraction has gained the ascendency; its motion then becomes accelerated by the centripetal power acting on the planet, until its velocity becomes equal to the projectile force with which it was first put in motion. Therefore it must continue to revolve in an elliptical orbit as before stated. And when these two forces are equal on the body in motion, they never act at right angles, but in such acute angles, that the planet is moving with such velocity, that the ascendency is instantly obtained. In order to make the projectile force balance the gravita ting power so exactly, as that the body may move in a circle, the projectile velocity of the body must be such as it would have acquired by gravity alone, in falling through half the radius of the circle. By the above mentioned law, bodies will move in all kinds of elliptical orbits whether long or short; if the spaces in which they move in the longer ellipses have so much the less projectile force impressed upon them in the higher parts of their orbits, and their velocities in coming down towards the sun, are so prodigiously increased by his attraction, that their centrifugal forces in the lower parts of their orbits are so great as to overcome the sun's attraction there, and cause them to ascend again towards the higher parts of their orbits; during which, the sun's attraction acting so contrary to the motions of those bodies, causes them to move slower and slower until the projectile forces are diminished, almost to nothing, and then they are again brought back by the sun's attraction as before. If the projectile forces of all the planets (and like wise those comets, whose mean distances from the sun have been ascertained,) were destroyed at their mean distances from the sun; their gravities would bring them down, so that Mercury would fall to the sun in 15 days and 13 hours; Venus in 39 days and 17 hours; the Earth or Moon in 64 days and 10 hours; Mars in 121 days; Jupiter in 290 days; Saturn in 767 days; and Herschel in 5406 days; the nearest comet within the orbits of the planets in 13,000 days; the middlemost in 23,000 days, and the outermost in 66,000 days. The Moon would fall to the earth in 4 days and 20 hours, Jupiters first moon would fall to him in 7 hours; his second in 15 hours; his third in 30 hours; and his fourth in 71 hours.Saturns first moon would fall to him in 8 hours, his second in 12 hours; his 3d in 19 hours; his 4th in 68 hours; his 5 in 336 hours. A stone would fall to the earth's centre if there were an hollow passage in 21 minutes and 9 seconds*. The rapid motions of the moons of Jupiter and Saturn round their primaries, demonstrate that these two planets have stronger attractive power than the earth; for the stronger that one body attracts another, the greater must be the projectile force, and consequently the force of the other body must be increased to * The squares of the times, that any planet would fall to the sun are as as the cubes of their distances: or multiply the time of a whole revolution by 0176766 the product will be the time in which the planet would fall to the sun. and yet this second Moon Jupiter, whilst our Moon What a prodigious at keep it from falling to its central planet. Jupiter's second moon is 124,000 miles farther from Jupiter than our Moon is from us; goes almost 8 times around goes once round the Earth. tractive power must the Sun then have, to draw all the planets and satellites of the system towards him, and what an amazing power must it have acquired to put all these planets and moons into such rapid motions at first. Amazing indeed to us, because impossible to be effected by the united strength of all the created beings in an unlimited number of worlds, but in no wise hard for the Almighty, whose Planetarium takes in the whole Universe. The Sun and planets mutually attract each other, the power by which they are thus attracted, is called Gravity. But whether this power be mechanical or not, is very much disputed. Observation proves that the planets disturb each other's motions by it, and that it decreases according to the squares of the distances of the Sun and planets; as great light which is known to be material, likewise does. Hence, Gravity should seem to arise from the agency of some subtle matter, pressing towards the Sun and planets, and acting like all mechanical causes, by contact. But when we consider that the degree or force of Gravity, is exactly in proportion to the quantities of matter in those bodies, without any regard to their magnitudes or quantities of surface, acting as freely on their internal as external parts, it appears to surpass the powers of mechan I |
