explain the phenomena of gravitation; we need to know precisely how the intensity of this attraction is affected by the comparative magnitude of the masses and by the distance of the masses from each other. Now the theory of Kepler in regard to gravity was correct as to the first of these points, namely that the intensity of this attraction was directly as the mass, but he was in error in regard to the second point, as he supposed that the intensity of the attraction was inversely as the distance, instead of what was subsequently found to be the fact, the square of the distance.
Once more, to estimate at its just value the part which Kepler performed in the discovery of the laws of gravitation, we should bear in mind, that an hypothesis, even if subsequently it be found to be correct, is of no authority until its truth be demonstrated. It may be of great importance, by way of suggestion, in directing the labors of subsequent inquirers, but the chief merit of the discovery of the truth is due to the individual who furnishes its demonstration. When this is done, and not before, that which was previously but an hypothesis takes its place among the recognized laws of nature.
As in Kepler's day, the tendency of bodies near the earth to fall toward its center and the motions of the heavenly bodies were regarded as phenomena of entirely different laws of nature, his views as to the physical cause of planetary motion next claim our attention. He supposed that the motion of the planets in their orbits was due to an influence emanating from the sun, but assuming that if this influence were an attractive force, similar to terrestrial gravity, its effect would be to cause the planets to fall toward the sun in straight lines, instead of the actual motion of revolution about the sun; he supposed that the emanation was of a corporeal nature, somewhat analogous to light; that as the sun revolved on its axis, this emanation revolved with it just as the spokes of a wheel when the hub revolves, and that the planets were swept along in their orbits by the revolution of this emanation—the force which caused them to move being a propulsion and not an attraction. As the hypothesis would seem to require that the times of revolution of all the planets should be the same, whereas they are different, the nearer performing their annual revolution in a time less than the more remote—he supposed that the density of the emanation diminished as its distance from the sun increased; that consequently its virtue, or propulsive energy, diminished in like manner, just as the intensity of light diminishes with the increase of the distance from the luminous center. This would account in a general way for the fact that the times of revolution of the planets nearer the sun are shorter than the times of revolutions of those more remote, but the precise difference in the observed times of revolution was not exactly that which would be required by the hypothesis. Moreover, he had observed
