The first certain gleam of this grand conclusion obtained by Newton, is said so to have overpowered him, that he was obliged to suspend his calculations, and call in the aid of a friend, to finish the last few arithmetical computations. He saw the important relations of the demonstration—the planets wheeling round the sun—the satellites round the planets—the far wandering comets returning to the source of light in obedience to the law of gravitation: a result sufficient to throw the successful discoverer into nervous excitement. It is clear that, if a body be projected into space, it will proceed in the direction of the original impulse, and with a uniform velocity, forever—supposing no obstacle to impede its course. But the combination of two antagonistic forces will produce a resulting motion in a diagonal direction.
Suppose the straight lines A B, to represent the direction in which the earth would travel under the influence of the projectile force, which launched it into universal space: the straight lines A S, are those it would describe at any point of its orbit, if surrendered to the influence of the sun’s attraction. The primitive impulse is, however, checked by the solar attraction, and the latter by the former; so, that while the earth—if abandoned to either—would describe A B, or A S, the effect of their joint influence incessantly acting is to deflect it from both, and produce a curved path. The cause perpetually operating, the effect is constant—and hence the formation of the terrestrial orbit; and the cause extending to the other bodies in the system, the planetary orbs are deflected from their natural rectilinear paths, and pursue a circuit round the common centre. The force of attraction is, however, proportional to the quantity of matter, and the proximity of the attracting body. Like light, the power of gravitation is weakened by diffusion, and diminishes as the square of the distance increases. This square is the product of a number multiplied by itself. A planet, therefore, twice our distance from the sun, will gravitate four times less than we do—the product of two multiplied by itself being four. Such is the great Law of Gravity, subject to the two conditions, that its force is directly as the mass of the bodies, and inversely as the square of the distance. It extends to the confines of the system, and acts as a mighty invisible chain to keep the primary bodies in brotherly relationship to each other, and in mutual subjection to the central luminary. And who can trace its operation without recognizing a Supreme Potentate, who appointed to the sun his place, launched the planets in the depths, obedient to a law which has preserved the family compact—originally established—unbroken through the long series of ages.