106 LECTURES 



astronomical science in the seventeenth century. And even then the 

 clearest and strongest intellects were slow in extricating themselves 

 from the trammels of the Aristotelian philosophy. 



It was not until the time of Kepler that any real progress was 

 made in discovering the true laws of planetary motion. He discarded 

 all dependence upon authority. His views were independent and 

 hold, but in many respects extremely defective and often whimsical. 

 Yet his labors will ever be regarded as forming a brilliant epoch in 

 the history of astronomy. 



It is true that Copernicus had established the true system of the 

 world, which fixes the sua in the centre ; Galileo had demonstrated 

 the laws of motion, in falling bodies, and laid the foundation of 

 dynamical science ; Tycho Brahe had, for a quarter of a century, most 

 diligently watched and recorded the motions of the heavenly bodies. 

 But neither of these illustrious astronomers had ventured any hypo- 

 thesis for explaining the cause of those motions. 



By the laborious comparison of the observations of Tycho, Kepler 

 discovered the three laws of planetary motion which bear his name, 

 and which constitute the ground work of physical astronomy. They 

 are familiar to every student. The first is, that the planets revolve in 

 elliptical orbits ; the second, that the radius-vector, or the line draivn 

 from the sun to the planet, describes equal areas in equal times, in what- 

 ever part of the orbit it is located ; the third, that the squares of the 

 periodic times are proportional to the cubes of the mean distances. This 

 third law may justly be regarded as one of the grandest inductions in 

 the whole range of physical science. But Kepler was still far from 

 having any just conception of the true physical cause of these motions. 



He rejected the crystalline spheres of the G-reeks and located in the 

 sun a kind of central virtue, by which the planetary motions were 

 maintained. But this force or vii^tue, for he uses both terms, must 

 have the efiect, as he perceived, not of drawing bodies directly toioard 

 the sun, but of moving them across or athwart this line of direction. 

 It must, in some way, carry bodies round the sun. 



In order to render this idea plausible he was obliged to introduce a 

 whirling fiuid which bore tbe planets round in the same manner that 

 a running stream carries a boat. This was substantially the same 

 hypothesis which, nearly half a century later, was propounded by 

 Des Cartes, and was known as his celebrated system of vortices. 



But, in addition to his other agencies, Kepler gravely believed and 

 taught that each planet was animated by a spirit which held to the 

 matter of the planet a relation analogous to that which the mind of 

 man holds to his body. This great animal spirit acted an important 

 part in giving to the planet the right direction. 



Nor can we be much surprised at this fancy when we find even the 

 great Newton gravely recording his suspicion, as he does in his Prin- 

 cipia, that " the spirit which constitutes the most subtile and best 

 portion of our atmosphere, and which is necessary to all life, is derived 

 from comets." 



But, unsuccessful as Kepler was in discovering the true physical 

 cause of the planetary motions — the law which yms underlying his 



