THE GREATEST PLANET. 187 



these objects must have some proper motion on its own 

 account, and can in no case be structurally a part of the 

 rigid globe of the planet. In the "Companion to the 

 Observatory " for 1886 tables will be found which show 

 the times at which both the red spot and the white spot 

 were to be expected during that year on the central meri- 

 dian of Jupiter. Considering that these objects are of 

 such an unstable material, we can hardly be surprised at 

 the editor's complaint that the motion of the white spot 

 has appeared of late to be irregular. 



It has sometimes been thought that the great red spot 

 may be a protuberance from the partially solid interior of 

 the planet, which has been forced up through the over- 

 lying stratum of clouds, and thus brought into visibility. 

 So long as it lasts it will provide us with a tolerably 

 definite point on Jupiter from which longitudes can be 

 measured, and thus take the place which Dawes' forked 

 bay provides for us on Mars. 



There is another instructive line of reasoning by which 

 we can demonstrate that the Jupiter which we see is 

 certainly not to be regarded as a solid or rigid body. Its 

 bulk is so enormous that it is fully 1,200 times as big as our 

 earth. So much for the measurement of the giant planet ; 

 now as to its weight. We have already explained in con- 

 nection with the satellites of Mars how it is feasible, by 

 observations of the satellites, to discover the mass of the 

 planet to which those satellites are appended. In the appli- 

 cation of this method to the determination of the mass of 

 Jupiter we have a choice of no fewer than four fine satel- 

 lites from which to make the observations. The mass oi 

 Jupiter can also be obtained by other methods of inquiry, 



