ON ASTRONOMY. 107 



laws — lie gave a great impulse to tlie spirit of inquiry. The energies 

 of all the ablest mathematicians and astronomers of the age were con- 

 centrated upon this problem. 



In the mean time other branches of science were throwing light 

 upon this. Gilbert, of England, had investigated the laws of mag- 

 netism, and in 1600 had published a work, remarkable for that period, 

 on the magnet. This work was known to Galileo and Kepler, and 

 was much esteemed by them. The attraction between two magnets 

 suggested the idea that some analogous force might exist between 

 other bodies. Gilbert explained the influence of the earth upon the 

 moon by regarding the earth as a great loadstone. In like manner, 

 he supposed the moon to have a reciprocal action upon the earth, 

 not only upon the matter of the earth but upon "certain subterra- 

 nean humors and s|)irits" which are drawn out and modified by the 

 moon's action. Galileo and Kepler, as Grant says, both professed 

 themselves to be much indebted to the views contained in Gilbert's 

 work upon the magnet. 



Kepler fully adopted the principle of some sort of attraction, but 

 how that force acted in maintaining the orbits he did not comprehend. 

 His great work on astronomy, the "Astronomia Nova," was published 

 in 1609. In this work he maintains that some sort of attraction 

 exists between all the planets of the sy8tem_, and that the planetary 

 motions are maintained by some sort of influence emanating from the 

 sun. Tie seemed to be constantly impressed with the idea that a cen~ 

 tripelal force alone would not keep a planet in motion. It must have 

 some force either pulling it or pushing along in its course. And hence 

 he was obliged, on this hypothesis, to make the sun-force propel a 

 planet in a direction perpendicular to the radius- vector. 



We may judge how slow was the progress of true ideas upon this 

 subject from the fact that the law of gravitation was not discovered 

 till considerably more than half a century after the publication of 

 Kepler's work, before mentioned. 



In the mean while the law of planetary attraction engaged the 

 attention of Galileo, of Borelli, of BouUiau, of Des Cartes, Cassini, 

 Huygheus, Hooke, Halley, Sir Christopher Wren, and finally of 

 Newton. Among these Christian Huyghens, by common consent, is 

 second only to the illustrious Newton. Huyghens, who preceded 

 Newton by a few years published a very remarkable work in 1671 on 

 the pendulum. In this work, entitled " DeHorologio Oscillatorio," 

 he inserts several important theorems respecting the motion of a 

 heavenly body, which is constantly drawn to a fixed centre Borelli, 

 of Pisa, had, a few years before, (1666,) published a work in which 

 he quite accurately describes the action of centrifugal and centripetal 

 forces, and shows that they would together cause a body to revolve in 

 a circular orbit. 



At this time, 1665, when Newton's attention was first drawn to the 

 subject, it had become a common opinion, though by no means a uni- 

 versal one, that the planets were retained in their orbits by a force 

 residing in the sun. And the great point at issue now was to deter- 

 mine what kind of an orbit it would produce, and how it varied with 

 the distance ; whether it was inversely as the distance, or inversely as 



