THE AGE OF THE EARTH 65 



to the first inhabitants of the globe as they are to the plants 

 and animals which live upon it now. 



The answer to this question hardly comes within the 

 province of the astronomers. Yet they were the first to 

 show that there is evidence of such a change in the move- 

 ments of the earth as must, when traced backwards, bring 

 us at last to a far limit for its inhabitableness. Astronomi- 

 cal observations prove that the rapidity with which the 

 earth rotates has sevsibly diminished within historic times. 

 Laplace showed that the relative velocity of the rotation of 

 the earth and of the orbit of the moon have changed. The 

 time of commencement of eclipses of the moon, the time, 

 that is to say, after the moon had risen before the eclipse 

 commenced, and of their duration, have been recorded with 

 accuracy since they were noted by the astronomers of Baby- 

 lon twenty-seven centuries ago, and from these records it is 

 clear that either the rate at which the moon travels has 

 increased or the rapidity of the earth's rotation has steadily 

 diminished. Laplace considered that the moon has hurried 

 while the earth has kept time, and he pointed out a cer- 

 tain cause (the progressive diminution of the excentricity of 

 the earth's orbit) which must produce an acceleration of the 

 moon's motion ; but Adams, after estimating the utmost 

 effect of this accelerating cause, found that it can only ac- 

 count for one-half of the discrepancy in time between the 

 moon and the earth. It is indisputably true that the earth 

 is losing its velocity of rotation. It is twenty-two seconds 

 later at the end of every century. Every day is therefore 

 longer by the fraction of a second than the corresponding 

 day of the year before. 



F,or this loss of time on the earth's part the moon is 

 chiefly responsible, since the attraction of the moon is the 

 main factor in producing tides, and the slowing of the 

 earth is due to the friction of its envelope of water. As 

 the earth rotates it tends to leave its; envelopes of water 

 and air behind it, because the attraction of the moon and 



