OUR CHIEF TIME-PIECE LOSING TIME. 41 



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 rota- 

 tion, getting 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, obtained 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 comparison of observations ex- 

 tending over 200 years has enabled the present writer 

 to obtain a value which he considers to lie one- 

 hundredth part of a second of the truth. This value 

 for Mars' rotation-period is 24 hours 37 minutes 22 '74 

 seconds. 



Here, then, we have a result so accurate, that at 

 some future time, it may serve to test the earth's 

 rotation-period. We have compared the rotation-rate 

 of our test-planet with the earth's rate during the 

 past 200 years ; and therefore, if the earth's rate 

 vary by more than one-hundredth of a second in the 

 next two or three hundred years, we shall or, rather, 

 our descendants will begin to have some notion of 

 the change at the end of that time. 



But, in the meantime, mankind being impatient, 

 and not willing to leave to a distant posterity any 

 question which can possibly be answered now, astro- 

 nomers have looked around them for information avail- 

 able at once on this interesting point. The search has 



