192 LIGHT SCIENCE FOR LEISURE HOURS. 



equator is considerable ; so that, as seen from the sun,, 

 the equator passes far to the north and far to the south 

 of the centre of the disc, during the summer of the 

 northern and southern hemispheres, respectively. We 

 should expect to find these changes accompanied \>y 

 corresponding changes in the position of the central 

 zone of clouds. Our terrestrial tropical cloud-zone,, 

 being sun-raised, follows the sun, passing north of the 

 equator during our northern summer, until at midsum- 

 mer it reaches the tropic of Cancer, and passing south 

 of the equator during the southern summer, until at 

 midsummer (December) it reaches the tropic of Capri- 

 corn. But instead of the mid-zone of Saturn behaving 

 in this way, it remains always equatorial. 



Another (the sixth) argument, and in my opinion 

 an argument altogether irresistible, is derived from the 

 changes which have taken place from time to time in 

 the outline of the planets Jupiter and Saturn, unless 

 observations made by most skilful astronomers, and 

 with instruments of considerable power, are to be 

 rejected as unworthy of trust. I refer in particular ,. 

 first to the observations by Admiral Smyth, Sir R. 

 Maclear, and Professor Peacock, of the reappearance of 

 the second satellite of Jupiter a few minutes after it 

 had apparently made its complete entry upon the 

 planet's disc at the beginning of a transit ; and, 

 secondly, to the fact that Sir W. and Sir J. Herschel, 

 Sir Gr. Airy, the Bonds and Coolidge in America, and 

 several of the Greenwich observers, have recognised the 

 occasional assumption by Saturn of what is commonly 



