THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 325 



the three laws which, since Newton and the promulgation 

 of the theory of gravitation, have immortalised the name of 

 Kepler. ( 50 ) Cosmical speculations, even such as are not 

 founded on observation, but only on faint analogies, then, 

 as is still often the case, arrested attention more than the 

 most important results of " calculating astronomy/' 



Having thus described the important discoveries which, 

 in so small a cycle of years, enlarged the knowledge of the 

 regions of space, I have still to recal the advances in physi- 

 cal astronomy which marked the second half of the great 

 century of which we are treating. The improvement of 

 telescopes occasioned the discovery of the satellites of 

 Saturn. Huygens, with an object-glass polished by himself, 

 first di?covered one of them (the sixth), on the 25th of 

 March, 1655, forty-five years after the discovery of Jupiter's 

 satellites. Prom a prejudice which Huygens shared with 

 several astronomers of the period, that the number of satel- 

 lites or secondary planets could not exceed that of the 

 larger or primary planets, ( 501 ) he did not seek to discover 

 any more of the satellites of Saturn. Pour of them, Sidera 

 Lodovicea, were discovered by Dominic Cassini : the seventh, 

 or outermost, which has great alternations of brightness, in 

 1671 ; the fifth in 1672 ; and the third and fourth in 1684, 

 with an object-glass of Campani's having a focal length of 

 100 136 feet. The two innermost, or the first and second 

 satellites, were discovered more than a century later (1788 

 and 17 8y), by William Herschel with his colossal telescope. 

 The second satellite offers the remarkable phsenomenon of 

 performing its revolution round the principal planet in less 

 than one of our days. 



Soon after Huygens' discovery of a satellite of Saturn, 



