OF ARTS AND SCIENCES. 319 



spots we have describetl in tliis paper. The period of each spot is the 

 mean of tlie number of rotations during which each one was individ- 

 ually observed. 



AiT'^oxiMATE Period of Rotation of the Marks observed, in tiieik 



Order of Decrease. 



Mean period of rotation obtained from 

 h. ni. s. 



This table shows conclusively that the different spots and mark- 

 ings studied in this paper did not return to the meridian in the same 

 interval of time, each having a distinct and separate period of rotation, 

 varying from Q** 55™ 38' to 2\ Not only each one had a distinct 

 period of rotation, but, again, it is shown by the other tables that the 

 very same spot had different periods of rotation, varying from day to 

 day, and this sometimes considerably. On some days the spots moved 

 rapidly, and made great jumps forward, while on others they moved 

 slowly, and even seemed to retrograde. For the last ten years I have 

 closely observed Jupiter, and the same phenomena have occurred ; 

 therefore the proper motion of the markings of the planet is not an 

 accident. In 1876, as already stated, the changes were much more 

 rapid, and on a larger scale, than they have been since. During that 

 year, not a spot or mark, save in one instance, could be recognized on 

 the following day. 



Since no marking which could be taken for reference on the surface 

 of Jupiter is absolutely fixed on the planet, it is impossible to obtain 

 the exact period of rotation of the solid nucleus of tliis body; and it is 

 therefore probable, if not absolutely certain, that no one has yet suc- 

 ceeded in obtaining it. Unless some great changes occur in the con- 

 stitution of the planet, it is not likely that astronomers' are soon to 

 obtain that exact period. Tlie period of rotation which we obtain 

 now is the period of clouds, similar in appearance to those of our 

 earth, floating in a dense atmosphere : the period is longer or shorter, 



