amount of energy, but the mechanical equivalent of heat opens a new aspect of the question.  All the celestial motions are vast potential stores of heat, and if checked or arrested, the heat would at once become manifest.  Could we imagine brakes applied to the surface of the sun and planets, so as to arrest, by friction, their motions upon their axes, the heat thus produced would be sufficient to maintain the solar emission for a period of one hundred and sixteen years.  As the earth is eight thousand miles in diameter, five and a half times heavier than water, and moves through its orbit at the rate of sixty-eight thousand miles an hour, a sudden arrest of its motion would generate a heat equal to the combustion of fourteen globes of anthracite coal as large as itself.  Should it fall into the sun, the shock would produce a heat equal to the combustion of five thousand four hundred earth-globes of solid coal,—­sufficient to maintain the solar radiation nearly a hundred years.  Should all the planets thus come to rest in the sun, it would cover his emission for a period of forty-five thousand five hundred and eighty-nine years.  It has been maintained that the solar heat is actually produced in this way by the constant collision upon his surface of meteoric bodies, but for the particulars of this hypothesis we must refer to the book itself.

Professor Tyndall opens the question in his volume respecting the share which different investigators have had in establishing the new theory of forces, and his observations have given rise to a sharp controversy in the scientific journals.  The point in dispute seems to have been the relative claims of an Englishman and a German—­Dr. Joule and Dr. Mayer—­to the honor of having founded the new philosophy.  Tyndall accords a high place to the German as having worked out the view in an a priori way with remarkable precision and comprehensiveness, while he grants to the Englishman the credit of being the first to experimentally establish the law of the mechanical equivalent of heat.  But his English critics seem to be satisfied with nothing short of an entire monopoly of the honor.  The truth is, that, in this case, as in that of many others furnished us in the history of science, the discovery belongs rather to an epoch than to an individual.  In the growth of scientific thought, the time had come for the evolution of this principle, and it was seized upon by several master-minds in different countries, who worked out their results contemporaneously, but in ignorance of the efforts of their fellow-laborers.  But if individual claims are to be pressed, and each man accorded his aliquot share of the credit, we apprehend that America must be placed before either England or Germany, and for the explicit evidence we need look no farther than the volume of Professor Tyndall before us.  The first clear connection and experimental proof of the modern theory was made by our countryman Benjamin Thompson,—­afterwards knighted as Count Rumford