52 LIGHT SCIENCE FOR LEISURE HOURS. 



Considered as a time-piece, what are the earth's 

 errors? Suppose, for a moment, that the earth was 

 timed and rated two thousand years ago, how much 

 has she lost, and what is her ( rate-error ? ' She has 

 lost in that interval nearly one hour and a quarter, and 

 she is losing now at the rate of one second in twelve 

 weeks. In other words, the length of a day is now 

 more by about one-eighty-fourth part of a second than 

 it was two thousand years ago. At this rate of change, 

 our day would merge into a lunar month in the course 

 of thirty-six thousand millions of years. But after a 

 while, the change will take place more slowly, and 

 some trillion or so of years will elapse before the full 

 change is effected. 



Distant, however, as is the epoch at which the 

 changes we have been considering will become effec- 

 tive, the subject appears to us to have an interest apart 

 from the mere speculative consideration of the future 

 physical condition of our globe. Instead of the recur- 

 rence of ever-varying, closely intermingled cycles of 

 fluctuation, we see, now for the first time, the evidence 

 of cosmical decay a decay which, in its slow progress, 

 may be but the preparation for renewed genesis but 

 still, a decay which, so far as the races at present sub- 

 sisting upon the earth are concerned, must be looked 

 upon as finally and completely destructive.* 



(From Chambers' s Journal, October 12, ]867.) 



* In the Quarterly Journal of Science for October 1866, a more 

 detailed but somewhat less popular account of the subject of the above 

 paper is presented. A few months earlier, a charmingly-written paper 

 on the same subject, from the pen of Mr. J. M. Wilson, of Rugby, had 



