1 86 IN STARRY REALMS. 



inference that it possesses nothing of a permanent cha- 

 racter on its surface. It is true that we search in vain on 

 the great planet for anything so definite as the Kaiser 

 Sea, or one or two other objects that might be mentioned 

 on Mars. But there is at least one mark on Jupiter which 

 has been continuously observed for several years, and is 

 known as the great red spot on Jupiter. In 1879 it had 

 attained remarkable distinctness and such permanence 

 that, though it has undergone no infrequent changes in 

 brightness and appearance, it is a feature on the planet at 

 the present time. A remarkable phenomenon in connec- 

 tion with the proof of the absence of anything like rigidity 

 on the surface of Jupiter is strikingly brought out by the 

 behaviour of the red spot and of a white spot in its 

 vicinity. The rotation of the red spot, as computed in 

 the Olsercatory for 1886, was accomplished in 9 hrs. 

 55 mins. 39 sees., while that of the white spot was 9 hrs. 

 50 mins. 10 sees. Thus it follows that the red spot took 

 5J minutes longer to complete its circuit of the planet 

 than was required for the white one. If the globe of 

 Jupiter were really rigid, and if the spots were permanent 

 objects on that globe, then, of course, the period of rota- 

 tion concluded from either spot must be the same as that 

 concluded from the other, and both must be equal to that 

 of the rotation of the planet on its axis. In fact, to 

 suppose that there could be any difference would be 

 similar to saying that the day which is twenty-four hours 

 long in London was five minutes less than twenty-four 

 hours in Paris. It is obvious, therefore, when we find 

 the objects on Jupiter differing by five minutes in the 

 periods they give for the rotation, that one or both of 



