40 LIGHT SCIENCE FOR LEISURE HOURS. 



not afford us exact information his real surface being 

 always veiled by his dense and vapour-laden atmo- 

 sphere. Saturn, Venus, and Mercury are similarly 

 circumstanced, and are in other respects unfavourable 

 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 vapours, 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- 

 lites to influence (perhaps appreciably in long in- 

 tervals of time) their rotation-movements. Venus and 

 Mercury are near the sun, and are therefore in this 

 respect worse off than the earth, whose rotation is in 

 question. Mars, on the other hand, farther removed 

 than we are from the sun, having also no moon, and 

 being of small dimensions (a very important point, be 

 it observed, since the tidal action of the sun depends 

 on the dimensions of a planet), is likely to have a 

 rotation-period all but absolutely constant. 



Herschel was rather unfortunate in his observations 

 of Mars. Having obtained a rough approximation 

 from Mars' rotation in an interval of two days this 

 rough approximation being, as it happened, only 

 thirty-seven seconds in excess of the true period, he 

 proceeded to take three intervals of one month each. 

 This should have given a much better value, but, as 

 it happened, the mean of the values he obtained was 



