130 SCIENTIFIC APPARATUS. 



application or other of one of the group (2). Let us take these 

 up in order. 



I. NATURE OF HEAT. 



What we now know to be the transformation of mechanical 

 energy into heat by friction has been habitually effected by savage 

 man from time immemorial. The converse transformation, that of 

 heat into work, dates back to the time of Hero at least. But the 

 knowledge that a certain physical process will produce a certain 

 result does not necessarily imply even a notion of the " Why ;" 

 and Hero as little imagined that in his QEolipile heat was converted 

 into work, as do savages that work can be converted into heat. 



That heat is one of the many forms of what we now call 

 Energy, was all but established by Rumford and Davy at the very 

 end of last century. It was first, however, clearly stated by Davy 

 in 1812. Rumford's observations on the heat generated in the 

 boring of cannon lead to an estimate of the mechanical value of 

 heat, which is only about 20 per cent, too great. 



The extensive, and exceedingly accurate, experiments of Joule 

 led, in 1843 and subsequent years, by processes depending directly 

 on friction, to numbers varying from 770 to 774 foot-pounds of 

 energy as the equivalent of one unit of heat on the Fahrenheit 

 scale. The number finally assigned by Joule (for the latitude of 

 Manchester) is 772, and it is almost certainly not in error by 

 anything approaching to i per cent. 



In 1853 Joule verified this result by means of a very accurate 

 determination of the specific heats of air, and a direct experi- 

 mental proof (given in 1845) that the heat developed by the 

 sudden compression of air is very nearly the equivalent of the 

 work expended. 



Direct measurements of the heat produced by the expenditure 

 of mechanical energy were made in various ways by Colding, in 

 1843 ; and have been repeated in many forms by Him, Regnault, 

 &c., since the publication (in 1849) of Joule's final result. 



