8 Historical Sketch of Improvements in [July, 



perfect vacuum, it would never cool at all. I consider the 

 experiments of Dulong and Petit to have overturned this opinion, 

 and to have shown that even in a vacuum a hot body would cool, 

 in consequence of the property which bodies have of radiating^. 

 heat. Indeed Mr. Leshe's opinion could be considered in no 

 other light than as a simple conjecture totally unsupported by 

 experimental proof. 



The investigation of the quantity of heat evolved by the 

 compression of air contained in this essay is very ingenious ; but 

 the reasoning, probably from his not condescending to make us 

 acquainted with the intermediate links, is not of such a kind as 

 to prove perfectly satisfactory to the reader. The same obser- 

 vations apply I conceive to Mr. Leslie's explanation of the origin 

 of heat from percussion and friction. The notion that the mean 

 temperature of the earth is perpetually increasing has been 

 -entertained by other persons as well as Mr. Leslie ; though I 

 am not aware that any other person has attempted to estimate 

 the rate, which, according to him, is about 1° in 800 years, or 

 to deduce from that rate the period which has elapsed since the 

 beginning of things. Nothing can exceed the ingenuity of this 

 whole discussion ; yet I cannot help being of opinion that it 

 exhibits an example of the improper application of mathematics 

 to a case that is not capable of bearing that science. 1 am not 

 myself disposed to admit the data from which Mr. Leslie has 

 calculated. Now it is needless to say that upon these data the 

 whole value of the discussion depends. The i-easoning respect- 

 ing the rate at which the temperature of the air diminishes as 

 "we ascend in the atmosphere is good, and I suspect had it been 

 published in 1793, it would have had the merit of originality. I 

 am not so much satisfied with the observations on climate, con- 

 tained in the last part of the paper, though I give full credit to 

 the sagacity and originality of the author. 



There is one assertion, however, of our author, which I must 

 notice, because I consider it as inaccurate. He says that no 

 sensible increase of temperature is ever observed when we 

 descend into the deepest mines Now the fact is that in most 

 cases a very marked mcrease of temperature is observed in deep 

 mines. In the copper mines of Cornwall, it is no uncommon 

 thing to find the air hot enough to raise the thermometer to 100°. 

 In the salt mines in Cheshire, the miners work without their 

 clothes, and rather complain of heat than cold. I had no ther- 

 mometer when I descended into the salt mine at Nantwich, but 

 from the feel, I should rate the temperature of the air between 

 80° and 90°. It will he seen from a table published by Mr. Bald 

 (Edin. Phil. Jour. i. 135), that the air and the water at the bot- 

 tom of the deep coal mines in Durham, Cumberland, Northum- 

 berland, and Stafi:brd, are from 12° to 19° higher than at the 

 surface of the earth. In shoit, wherever the subject has been 

 investigated, it has been uniformly observed that the mean tem- 



