Page:Popular Science Monthly Volume 15.djvu/420

406 of heat, in the year 1842, with the resources he had, and the exactness which he attained, is one of the most marvelous exploits in the whole history of science, is incomparably his greatest achievement, and is sufficient alone to place him in advance of all the thinkers who have devoted themselves to this great research. And we apprehend that this would have long ago been conceded but for the rival claims of Dr. Joule to this discovery. It is admitted on all hands, and even by Mayer himself, that Joule's laboratory processes were necessary and invaluable in completing the work, and placing this truth upon its firm and experimental basis. With great patience and skill he worked out the law of the mechanical equivalence of heat, as a demonstration that all men can verify, and, by the award of the whole scientific world, that law is permanently connected with his name. But Joule's results were reached only in 1849, while Mayer had arrived at the same result by other methods in 1842. What was it that both men were driving at? It was the working out of a great relation, or the establishment of a universal truth of nature. . Mayer reached it, by using the data that science had created for him. He got it first, he got it independently, and he got it exactly, or within a small fraction of the expression arrived at by Joule after six years of subsequent experiment. Mayer was the pioneer, the revealer, the creator of the theory, and Joule the verifier of his work. That verification was required and has made the name of Joule immortal; but who will compare 'it with that master stroke of genius by which from scanty materials the great truth was first independently seized and formulated? In 1849 Dr. Joule fixed the exact mechanical equivalent of heat after many laborious experiments, at 772 foot-pounds. Seven years previously Dr. Mayer pursued a method which gave the mechanical equivalent of heat as 771·4 foot-pounds.

It was alleged by Thomson and Tait, as we have seen, that Mayer's method had been adopted by the Frenchman Séguin, three years earlier, and that he anticipated the German in deducing the mechanical equivalent of heat. Séguin, in 1839, published a work on the steam engine, in Paris; and that work contains a table on the relations of pressures, temperatures, and mechanical effects of steam, from which it was alleged that the mechanical equivalent of heat may be inferred. But the widest discrepancies existed among the interpretations of these tables by different authorities. Upon a careful investigation of the subject Professor Tyndall found that Séguin's and Mayer's numerical results did not refer to the same things at all, and that Séguin's tables did not attempt to give the mechanical equivalent of heat. Professor Tyndall says: "It is only necessary, however, to read the foregoing pages to see that Mayer and Séguin are speaking of two totally different things; that the degrees of the one are not the degrees of the other; that the 'temperatures correspondantes' of the latter, which refer to his compressed steam, are not thermal units at all, and