L. Page — A Century's Progress in Physics. 311 



ing momentum to the reciprocating parts of a steam 

 engine only to check the motion later on in the stroke. 

 Finally Isaac Doolittle (14, 60, 1828), of the Bennington 

 Iron Works, ends the discussion by the pertinent remark : 

 "If there be, as is contended by one of your correspond- 

 ents, a loss of more than one third of the power, in trans- 

 forming an alternating rectilinear movement into a 

 continuous circular one by means of a crank, I should 

 like to be informed what would be the effect if the propo- 

 sition were reversed, as in the case of the common 

 saw mill, and in many other instances in practical 

 mechanics." 



A realization of the equivalence of heat and mechani- 

 cal work did not come until the middle of the century, in 

 spite of the conclusive experiments of the American 

 Count Rumford and the English Davy before the year 

 1800. So firmly enthroned was the caloric theory, 

 according to which heat is an indestructible fluid, that 

 evidence against it was given scant consideration. In 

 fact the success of the analytical method introduced by 

 Fourier in 1822 for the solution of problems in conduc- 

 tion of heat only added to the difficulties of the adherents 

 of the kinetic theory. But recognition of heat as a form 

 of energy was on the way, and when it came it made its 

 appearance almost simultaneously in half a dozen differ- 

 ent places. Perhaps Robert Mayer of Heilbronn was 

 the first to state explicitly the new principle. His paper 

 "On the Forces of Inorganic Nature" was refused 

 publication in Poggendorff 's Annalen, but fared better at 

 the hands of another editor. During the next few years 

 Joule determined the mechanical equivalent of heat 

 experimentally by a number of different methods, some 

 of which had already been devised by Carnot. Of those 

 he used, the most familiar consists in churning up a 

 measured mass of water by means of paddles actuated by 

 falling weights and calculating the heat developed from 

 the rise in temperature. However, the work of the 

 young ^Manchester brewer received little attention from 

 the members of the British Association before whom it 

 was reported until Kelvin showed them its significance 

 and attracted their interest to it. Meanwhile Helmholtz 

 had completed a very thorough disquisition on the con- 

 servation of energy not only in dynamics and heat but in 

 other departments of physics as well. His paper on 



