50 LIGHT SCIENCE FOR LEISURE HOURS. 



comparison of observations extending over 200 years 

 lias enabled the present writer to obtain a value winch 

 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, w r e have a result so accurate, that at 

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

 tion-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 de- 

 scendants will begin to have some notion of the 

 change at the end of that time. 



But, in the mean time, mankind being impatient, 

 and not willing to leave to a distant posterity any 

 question which can possibly be answered now, astron- 

 omers have looked around them for information 

 available at once on this interesting point. The search 

 has not been in vain. In fact, we are able to an- 

 nounce, with an approach to positiveness, that our 

 great terrestrial timepiece is actually losing time. 



In our moon we have a neighbor which has long 

 been in the habit of answering truthfully questions 

 addressed to her by astronomers. Of old, she told 

 Newton about gravitation, and when he doubted, and 

 urged contradictory evidence offered as men in his 

 time supposed by the earth, she set him on the right 



