34 LIGHT SCIENCE FOR LEISURE HOURS. 



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 

 extending over 200 years has enabled me to obtain 

 a value which I consider to lie within one-hun- 

 dredth part of a second of the truth. This value 

 for Mars' rotation-period is 24 hours 37 minutes 22'73 

 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 ques- 

 tion which can possibly be answered now, astronomers 

 have looked around them for information available at 

 once on this interesting point. The search has not 



