OUR CHIEF TIME-PIECE LOSING TIME. 33 



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 satellites to influ- 

 ence (perhaps appreciably in long intervals 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 dimen- 

 sions (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 abso- 

 lutely 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 chanced, 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 



D 



