﻿394 Scientific Intelligence. 



be very hazardous until we know more precisely the nature of the 

 thing changed. 



When looking at the astronomical theory of geological climate 

 as a whole, one cannot but admire the symmetry and beauty of 

 the scheme, and nourish a hope that it may be true ; but the 

 mental satisfaction derived from our survey must not blind us to 

 the doubts and difficulties with which it is surrounded. 



And now let us turn to some other theories bearing on this 

 important point of geological time. 



Amongst the many transcendent services rendered to science 

 by Sir William Thomson, it is not the least that he has turned 

 the searching light of the theory of energy on to the science of 

 geology. Geologists have thus been taught that the truth must 

 lie between the cataclysms of the old geologists and the uniformi- 

 tarianism of forty years ago. It is now generally believed that 

 we must look for a greater intensity of geologic action in the 

 remote past, and that the duration of the geologic ages, however 

 little we may be able mentally to grasp their greatness, must bear 

 about the same relation to the numbers which were written down 

 in the older treatises on theology, as the life of an ordinary man 

 does to the age of Methuselah. 



The arguments which Sir William Thomson has adduced in 

 limitation of geological time are of three kinds. I shall refer 

 first to that which has been called the argument from tidal fric- 

 tion ; but before stating the argument itself it will be convenient 

 to speak of the data on which the numerical results are based. 



Since water is not frictionless, tidal oscillations must be subject 

 to friction, and this is evidenced by the delay of twenty-four to 

 thirty-six hours which is found to occur between full and change 

 of moon and spring-tide. An inevitable result of this friction is 

 that the diurnal rotation of the earth must be slowly retarded, 

 and that we who accept the earth as our timekeeper must accuse 

 the moon of a secular acceleration of her motion round the earth, 

 which cannot be otherwise explained. It is generally admitted 

 by astronomers that there actually is such an unexplained secular 

 acceleration of the moon's mean motion. 



No passage in Thomson and Tait's " Natural Philosophy " has 

 excited more general interest than that in which Adams is quoted 

 as showing that, with a certain value for the secular acceleration^ 

 the earth must. in a century fall behind a perfect chronometer, set 

 and rated at the beginning of the century, by twenty-two sec- 

 onds. Unfortunately this passage in the first edition gave an 

 erroneous complexion to Adams's opinion, and being quoted 

 without a statement of the premises, has been used in popular 

 astronomy as an authority for establishing the statement that the 

 earth is actually a false timekeeper to the precise amount speci- 

 fied. 



In the second edition (in the editing of which I took part) this 

 passage has been rewritten, and it is shown that Newcomb's 

 estimate of the secular acceleration gives only about one-third of 



