OUR CHIEF TIMEPIECE LOSING TIME. 49 



lites to influence (perhaps appreciably in long intervals 

 of time) their rotation -movements. Yenus and Mer- 

 cury 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 dimen- 

 sions of a planet), is likely to have a rotation-period 

 all but absolutely constant. 



Herschel was rather unfortunate in his observa- 

 tions of Mars. Having obtained a rough approxima- 

 tion 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 

 forty-six seconds too great. He then took a period of 

 two years, and being misled by the erroneous values 

 he had already obtained, he missed one rotation, get- 

 ting a value two minutes too great. Thirty years ago, 

 two German astronomers, Beer and Madler, tried the 

 same problem, and taking a period of seven years, ob- 

 tained a value which exceeds the true value by only 

 one second. Another German, Kaiser, by combining 

 more observations, obtained a value which is within 

 one-fifteenth of a second of the true value. But a 



