48 LIGHT SCIESTCE FOR LEISURE HOURS. 



Sir William Ilerschel, who early saw the impor- 

 tance of the subject, suggested another method. Some 

 of the planets rotate in such a manner, and bear such 

 distinct marks upon their surface, that it is possible, 

 by a series of observations extending over a long inter- 

 val of time, to determine the length of their rotation- 

 period within a second or two. Supposing their rota- 

 tion uniform, we at once obtain an accurate measure 

 of time. Supposing. their rotation not uniform, we 

 obtain (1) a hint of the kind of change we are look- 

 ing for ; and (2), by the comparison of two or more 

 planets, the means of guessing how the variation is to 

 be distributed between the observed planets and our 

 own earth. 



Unfortunately, it turned out that Jupiter, one of 

 the planets from which Herschel expected most, does 

 not afford us exact information his real surface beino; 



^ 



always veiled by his dense and vapor-laden atmosphere. 

 Saturn, Yenus, and Mercury, are similarly circum- 

 stanced, and are in other respects unfavorable objects 

 for this sort of observation. Mars only, of all the 

 planets, is really available. Distinctly marked (in 

 telescopes of sufficient power) with continents and 

 oceans, which are rarely concealed by vapors, this 

 planet is in other respects fortunately situated. For it 

 is certain that whatever variations may be taking 

 place in planetary rotations must be due to external 

 agencies. Now, Saturn and Jupiter have their satel- 



