MODERN SCIENCE. 199 



and observed in the camera obscura the transit of this planet 

 (May 3, 1661); measured its diameter; determined the 

 rotation of the sun on its axis to take twenty-seven 

 days, after assiduously observing the sun's spots. He in- 

 vented the polemoscope, for observing the movements of an 

 army out of sight. 



1619 41. Horrocks was the first, with his friend Crab- 

 tree, to observe the TRANSIT OF VENUS (over the sun's disk), 

 December 4, 1639, ky means of a telescope. 



1625 1712. Cassini discovered comets to belong to our 

 planetary system, and in some respects to partake of the 

 nature of planets a somewhat hasty conclusion ; he calculated 

 the orbits of a few ; measured the obliquity of the ecliptic ; 

 also the parallax of the sun ; also horizontal refraction ; 

 showed Jupiter to revolve on its axis, and its rotation to take 

 9 hours 56 minutes observing at the same time the 

 flattening of that planet at the poles, a discovery precious to 

 Newton ; discovered FOUR OF SATURN'S MOONS ; and also the 

 ROTATION OF MARS ; drew up the EPHEMERIDES of Jupiter's 

 satellites ; originated a method for determining parallaxes 

 still in use ; and finally studied the zodiacal light, the nature 

 of which remains as yet undetermined. 



162995. Huygens, by means of a ten feet long telescope 

 of his own making, was the first to discover one of SATURN'S 

 SATELLITES (Titan), and also to detect Saturn's rings to be 

 detached from the planet (1659) ; he also discovered the great 

 non-resolvable NEBULA OF ORION. 



16381703. Hooke determined the PARALLAX OF STARS 

 by a surer way than Galileo had proposed (1660) ; approached 

 in a remarkable manner the discovery of universal gravitation, 

 after making the earth's motion a subject of deep study and 

 calculation. 



1642 1727. Newton applying Galileo's principles of 

 terrestrial dynamics to the Universe, demonstrated THE LAWS 

 OF universal attraction and GRAVITATION his second and 

 greatest discovery a feat unparalleled in the annals of science, 

 sublime alike for its magnitude and for the labours it involved. 

 Before dealing with the problem, Newton had to resolve, 

 I. the force according to which a body falls; II. the exact 



