174 HERSCHEL AND HIS WORK 



considerable atmosphere. For besides the permanent 

 spots on its surface, I have often noticed," he says, 

 "occasional changes of partial bright belts and also 

 once a darkish one in a pretty high latitude. And 

 these alterations we can hardly ascribe to any other 

 cause than the variable disposition of clouds and 

 vapours floating in the atmosphere of that planet." 

 From the fact that the dark belts or spots and the 

 red colour of Mars manifestly belong to the surface of 

 the planet, we may accept Herschel's idea "that its 

 inhabitants probably enjoy a situation in many respects 

 similar to ours." It has been shown in our own day 

 that the vapour of water, and with that we may 

 associate clouds, is present in the atmosphere of Mars. 

 But there is reason to believe that the atmosphere of 

 Mars is comparatively rare. 



Jupiter was not one of the planets from which 

 Herschel reaped an ungathered harvest. The field 

 had been so thoroughly worked by others in searching 

 for a method of easily discovering the longitude at 

 sea, that it does not seem to have presented the same 

 attractions to him as other planets did. A paper 

 which he wrote on Jupiter in 1797 and he wrote no 

 other gives many curious quotations from his journal 

 regarding the planet and its satellites. So minute are 

 the discoveries made of change of colour and apparent 

 size of the satellites that if the Red spot, detected on 

 the planet in 1878, had been visible in his day, he 

 could scarcely have failed to see it. The bands or 

 belts on the body of the planet, the white and dark 

 spots they showed, the length of day they indicated, 

 and the rotation of the four satellites round their 

 primary were the principal points attended to by 



