ii 6 POPULAR LECTURES AND ADDRESSES. 



earth. No natural operation is thoroughly revers- 

 ible, and therefore, every plutonic action involves 

 something in the way of dissipation of energy. 

 But the grand and awful phenomena of volcanoes 

 and earthquakes, results of abnormal plutonic 

 activity, give rise probably to much less dissipation 

 of energy, summed for all parts of the earth from 

 age to age, than the continual silent action of the 

 conduction of heat outwards, the amount of which 

 we are able to estimate in a thoroughly definite 

 manner. Thus we find that from year to year the 

 earth, at the present time, is parting with heat at 

 the rate of 92 horse-power l per square kilometre.' 2 

 That is to say, from a square metre of surface the 

 loss of energy is at an average rate of seven mctrc- 



1 "One horse-power" is a rale of performing work equal to 

 (33,000 foot pounds, or) 4*563 metre-Ions per minute ; the Krencli 

 ton of IOOO kilogrammes understood, beini^ '9842 of the British 

 ton. 



" The kilometre is '62138 of that very inconvenient measure, the 

 British statute mile. The square kilometre is 247*11 of that, if pos- 

 sible- \\or>e measure, the acre. Experts can tell how many square 

 yards are in an acre : but of all the men in England accustomed to 

 reckon their land in acres, and to state, or read, or hear reckonings 

 of political statistics in square miles, very few could readily answer 

 the question, I low many acres are there in a square mile ? 



