72 POPULAR LECTURES AND ADDRESSES. 



I have given elsewhere l of the present state of 

 deep underground temperatures, and by taking 

 I(I ,H, UO - as the vertical contraction per degree 

 centigrade of cooling in the earth's crust, that the 

 gain of time on this account by the earth, regarded 

 as a clock, must be extremely small, and may even 

 not amount to more in a century than. -^ of a 

 second or ^ O ir f tne amount estimated above as 

 conceivably due to tidal friction. 



J ' Secular Cooling of the Earth," Transactions of Uic Royal 

 Society of Edinburgh^ 1862 ; and Philosophical Magazine, January, 

 1863. 



