48o POPULAR SCIENCE MONTHLY. 



explain the phenomena of gravitation; we need to know precisely 

 how the intensity of this attraction is affected by the comparative mag- 

 nitude 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, bnt 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 per- 

 formed in the discovery of the laws of gravitation, we should bear in 

 mind, that an hypothesis, even if subsequently it be found to be cor- 

 rect, is of no authority until its truth be demonstrated. It may be of 

 great importance, by way of suggestion, in directing the labors of subse- 

 quent 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 sup- 

 posed that the motion of the planets in their orbits was due to an in- 

 fluence emanating from the sun, but assuming that if this influence 

 were an attractive force, similar to terrestrial gravity, its effect would 

 be to cause tlie 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 attrac- 

 tion. As the hypothesis would seem to require that the times of revolu- 

 tion 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 dimin- 

 ished 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 



