24 THE TIDES. 



ON THE EFFECT OF THE TIDES ON THE LENGTH OF 

 THE DAY. 



1. Historical. 



In the year 1754 the Berlin Academy proposed, as the 

 subject for a prize essay, the question, " Does any Cause 

 exist tending to Eetard the Rotation of the Earth? " What 

 the result of the competition was I do not know ; but the 

 question led to the publication by Kant of a short essay, 

 in which he suggested that such a retarding cause existed 

 in the tides. He worked out this suggestion in a rough 

 way, there being, as he truly said, no ascertained data on 

 which any trustworthy calculation could be built. 



Laplace examined the question from the historical side, 

 with the help of the records of ancient eclipses, and came to 

 the conclusion that the period of rotation had not altered. 



Eecently, in consequence of the improvement of the 

 lunar tables, astronomers have seen reason to re-open the 

 question. It has been inferred from the records of ancient 

 eclipses that the day is lengthening at the rate of one 

 second in two hundred thousand years. At first sight 

 this may seem to be an amount too small to leave any 

 trace in history. It must be remembered, however, that 

 in calculating what part of the earth's surface came into 

 the shadow of a given total eclipse, say 2500 years ago, we 

 have to "unwind" 2500 times 365i (= 913,125) rotations, 

 and a difference amounting to an eightieth of a second 

 between the first and last of these would in the whole 

 period have a very considerable effect.* M. Delaunay 



* About 100 minutes: see Ball, "Elements of Astronomy," p. 377. 



