16 POPULAR LECTURES AND ADDRESSES. 



far as I know, that view was first given by 

 him. 



1 6. It is impossible, with the imperfect data we 

 is as to the tides, to calculate how much their 

 effect in diminishing the earth's rotation really is. 

 But even from such data as those referred to in 

 n, it can be shown that the tidal retardation 

 of the earth's rotation must be something very 

 sensible. Still, it is unsatisfactory to be in the 

 position of asserting that we know there must be 

 a retardation (we cannot tell how much) and then 

 to be told, in opposition to that theory, that 

 observations of ancient eclipses make it certain 

 that the length of the day has not varied by one 

 ten-millionth part of twenty-four hours from 721 

 years before the Christian era. 1 The calculation 

 was first made by Laplace. It depended in part 

 on the historical facts of two eclipses of the moon, 

 seen in Babylon, one of them March 19, 721 B.C., 

 which was first perceived " when one hour after 

 her rising was full}' past;" 2 the other on the 



./. ran it/it,; Sec. 433, Vol. ii., Ed. 1833. 

 '-' Dunthorne " On the Acceleration of the Moon," Phil. Trans 



(Million'., A/'t-id^fment, Vol. ix. ) 



