MARS AS A WORLD. 169 



were not long in forthcoming, and Professor Hall himself 

 described his great work in a complete memoir published 

 at Washington in the following year. It is to this paper 

 that we turn for full information on the subject. 



It has been often said that the astronomer's best assis- 

 tant is his wife, but perhaps few of them have the same 

 confession to make as Professor Asaph Hall does when he 

 tells his readers that at first he thought the prospect of 

 discovering new satellites to Mars was very discouraging. 

 Other astronomers, and skilful astronomers, too, had tried 

 to find the satellites, and they had not been successful. 

 Why, then, should he hope to succeed in an enterprise 

 which had been already pronounced desperate ? He tells 

 us that he felt this so keenly that he might have aban- 

 doned the search had it not been for the encouragement 

 of his wife. 



In 1877 the planet approached exceptionally close to the 

 earth, and this it was which indicated to Professor Hall 

 that the right time for making the search had arrived. In 

 this respect he, of course, only enjoyed the same facility 

 which every other astronomer possessed of choosing the 

 most favourable moment for making the attempt. 



There were, however, two circumstances which were 

 greatly in favour of the success of the search which Pro- 

 fessor Hall was enabled to make. In the first place, the 

 United States Naval Observatory is situated in the pure 

 skies of Washington, and the more northward latitude 

 of the observatory gives it also a distinct advantage over 

 the similar observatories in Europe. Another factor in 

 Professor Hall's success was the use which he enjoyed of 

 a superb object glass, two feet in diameter. This rnagni- 



