PORTION OF THE COSMOS. - THE PLANETS. 377 



Jupiter, including his four satellites, as now known to us, 

 is i o \-T~STW whilst in 1824 Laplace still considered it 



The rotation of Jupiter is performed, according to Airy, 

 in 9 h , 55 m , 21 Sf 3, mean solar time. Dominique Cassini, in 

 1665, had found it to be between 9 h 55 m and 9 h 56 m , by 

 means of a spot which continued to be visible on the disk 

 of the planet for several years, and even as late as 1691, 

 preserving always the same colour and outline ( 6l6 ). The 

 spots seen on Jupiter are, in most cases, darker than the 

 streaks or bands called Jupiter's belts. They do not, how- 

 ever, appear to belong to the actual surface of the planet ; 

 for it has occasionally been found that some spots, near the 

 poles more particularly, gave a different time of rotation 

 from that given by others in the equatorial regions. Ac- 

 cording to a very experienced observer Heinrich Schwabe, 

 at Dessau the dark, more definitely bounded, spots in the 

 two grey belts bordering the Equator have been seen for 

 several successive years exclusively in one of the belts only, 

 at one time in the southern, and at another in the northern 

 belt. The process of formation of these spots is therefore 

 subject to change in respect of place. Occasionally (it was 

 so, according to Schwabe's observations, in November 1834) 

 the spots of Jupiter, viewed with a magnifying power of 280 

 in a Fraunhofer's telescope, resemble small spots on the Sun 

 having nuclei surrounded by penumbras ; but even then 

 their degree of blackness is less than that of the shadows 

 of the satellites. The nucleus is probably a part of the 

 body of the planet itself; and when the atmospheric opening 

 through which it is seen maintains its place above the same 



