184 IN STARRY REALMS. 



instruments have been directed to the sky. Of recent 

 years the powers of great instruments have been devoted 

 to the study of Jupiter, and consequently we have at 

 our disposal an ample body of facts and an extensive col- 

 lection of drawings and measurements. 



The appearance of the planet in a telescope of any reason- 

 able pretensions generally shows several markings on his 

 surface (see Fig. 18). Especially noticeable under ordinary 

 circumstances are a pair of belts parallel to the planet's 

 equator, one to the north and the other to the south. These 

 features are well represented in the cut, which delineates 

 a series of telescopic appearances of the planet. As we 

 watch the globe from hour to hour, various changes in 

 its appearance force themselves on our notice. No doubt 

 these are partly to be accounted for by the rotation of the 

 planet on its axis. Certain features gradually pass away 

 into invisibility on one side, while new features come in 

 on the other. Even the actual appearance of them at the 

 centre of the disc is more or less affected by the varying 

 degrees of foreshortening under which they are exposed. 

 But after all allowance has been made for such mere 

 apparent changes, it is speedily seen by the attentive 

 observer that the actual marks themselves are not perma- 

 nent, that they are waxing and waning, now entirely 

 disappearing, while ever and anon fresh features break 

 forth. Even the two belts themselves are not constant ; 

 the uniformity of the belts is often interrupted, while 

 their margins undergo considerable changes. Sometimes, 

 indeed, the belts are not to be discerned at all, and some- 

 times, instead of merely a pair of belts, the globe reveals 

 several. The longer the surface of Jupiter becomes 



