ON GEOLOGICAL TIME. 37 



22nd December, 313 years B.C., which was first 

 perceived " half an hour before the end of night," 

 and which, though now known to have lasted only 

 about an hour and a half, had not come to an end 

 when the moon set. The rotation of the earth can- 

 not have experienced much retardation during these 

 2700 years, or else the moon-rise must have taken 

 place after instead of before the beginning of the 

 first of those eclipses ; and it cannot have ex- 

 perienced much acceleration, 1 or else the moon 

 must have set at Babylon before the second 

 mentioned eclipse commenced, which, therefore 

 could not have been seen from that place. But 

 Dunthorne showed that these records and various 

 observations regarding many other less ancient 

 eclipses all agree in demonstrating the cor- 

 rectness of a suspicion which Halley had raised 



1 As regards the earth's rotation, it seems to have been only 

 acceleration (due to cooling and shrinking) that was suspected until 

 Kant and others showed that the tides must produce retardation. 

 But Laplace proved, by calculations founded on Fourier's theory of 

 the conduction of heat (not at all on astronomical data), that the 

 acceleration by shrinking on account of cooling cannot have shortened 

 the day by as much as i-3Ooth of a second of time ; that is to say 

 by about a twenty-five millionth of its own amount in the last 2,000 

 years. 



