MARS AS A WORLD. 179 



rellous complexities of its motion, and to assign to each 

 of the causes of disturbance its due share of responsibility. 



It thus comes to pass that we learn how much Mars 

 affects the path of the earth, and hence we are able to dis- 

 cover the mass of the planet itself. Is not this a mar- 

 vellously recondite inquiry ? Yet it shows the perfection 

 to which astronomy, both theoretical and practical, has in 

 fact advanced, when I have to add, in conclusion, that the 

 mass of the planet Mars, discovered by this extremely 

 delicate and elaborate process, was found to be practically 

 coincident with the accurate value speedily and surely 

 proved when once the satellites had become known. 



A curious literary point was soon remarked in connec- 

 tion with the satellites of Mars. "We should not naturally 

 turn to " Gulliver's Travels " as an authentic manual of 

 astro'nomy, yet, strange to say, in that remarkable work 

 the astronomers in the flying Island of Laputa are 

 found to have already anticipated Professor Hall. These 

 people, we are assured, "are very bad reasoners, and 

 vehemently given to opposition, unless when they happen 

 to be of the right opinion, which is seldom the case." 

 Nevertheless, we learn that they have " discovered two 

 lesser stars or satellites, which revolve about Mars, 

 whereof the innermost is distant from the centre of the 

 primary planet exactly three of his diameters, and the 

 outermost five. The former revolves in the space of ten 

 hours, and the latter in twenty-one and a half." Thia 

 was certainly a marvellous anticipation of the actual facts 

 of the case. The astonishing feature is that the periodic 

 time attributed by Swift to the interior planet, should 

 have been so near correct. 



