When a moving body is stopped, its force is not annihilated, but simply takes another form. When the sledge-hammer strikes the leaden bullet and comes to rest, the mechanical force is not destroyed, but is simply converted into heat; and if all the heat produced could be collected, it would be exactly sufficient, when reconverted into mechanical force, to raise the hammer again to the height from which it fell. So, when bodies are rubbed together, their surface-particles are brought into collision, mechanical force is destroyed, and heat appears,—the heat of friction. The conversion of heat into mechanical motion, and of that motion back again into heat, may be familiarly illustrated in the case of a railway-train. The heat generated by combustion in the locomotive is converted into motion of the cars. But when it is desired to stop the train, what is to be done? Its mechanical force cannot be annihilated; it can only be transmuted; and so the brakes are applied, and the train brought to rest by reconverting its motion into heat, as is manifested by the smoke and sparks produced by the friction. Now, as heat produces mechanical motion, and mechanical motion heat, they must clearly have some common quality. The dynamical theory asserts, that, as they are both modes of motion, they must be mutually and easily convertible. When a moving mass is checked or stopped, its force is not annihilated, but the gross, palpable motion is infinitely subdivided and communicated to the atoms of the body, producing increased vibrations, which appear as heat. Heat is thus inferred to be, not a material fluid, but a motion among the ultimate atoms of matter.
The acceptance of this view led to the highly important inquiry, What is the equivalent relation between mechanical force and heat? or, how much heat is produced by a definite quantity of mechanical force? To Dr. Joule, of Manchester, England, is due the honor of having answered this question, and experimentally established the numerical relation. He demonstrated that a one-pound weight, falling through seven hundred and seventy-two feet and then arrested, produces sufficient heat to raise one pound of water one degree. Hence this is known as the mechanical equivalent of heat, or "Joule's Law."
The establishment of the principle of correlation between mechanical force and heat constitutes one of the most important events in the progress of science. It teaches us that the movements we see around us are not spontaneous or independent occurrences, but links in the eternal chain of forces,—that, when bodies are put in motion, it is at the expense of some previously existing energy, and that, when they come to rest, their force is not destroyed, but lives on in other forms. Every motion we see has its thermal value; and when it ceases, its equivalent of heat is an invariable result. When a cannon-ball strikes the side of an iron-plated ship, a flash of light shows that collision has converted the motion of the ball into intense heat, or when we jump from the table to the floor, the temperature of the body is slightly raised,—the degree of heat produced in both cases being ascertainable by the application of Joule's law.
The principle thus demonstrated has given a new interest and a vast impulse to the science of Thermotics. It is the fundamental and organizing conception of Professor Tyndall's work, and in his last chapter he carries out its application to the planetary system. The experiments of Herschel and Pouillet upon the amount of solar heat received upon the earth's surface form the starting-point of the computations. The total amount of heat received by the earth from the sun would be sufficient to boil three hundred cubic miles of ice-cold water per hour, and yet the earth arrests but 1/2,300,000,000 of the entire thermal force which the sun emits. The entire solar radiation each hour would accordingly be sufficient to boil 700,000,000,000 cubic miles of ice-cold water! Speculation has hardly dared venture upon the source of this stupendous 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 by the Elector of Bavaria. He went to Europe in the time of the American Revolution, and, devoting himself to scientific investigations, became the founder of the Royal Institution of Great Britain. Davy was his associate, and, so far as the new views of heat are concerned, his disciple. He exploded the notion of caloric, demonstrated experimentally the conversion of mechanical force into heat, and arrived at quantitative results, which, considering the roughness of his experiments, are remarkably near the established facts. He revolved a brass cannon against a steel borer by horse-power for two and one-half hours, thereby generating heat enough to raise eighteen and three-fourths pounds of water from sixty to two hundred and twelve degrees. Concerning the nature of heat he wrote as follows, the Italics being his own:—"What is heat? Is there any such thing as an igneous fluid? Is there anything that with propriety can be called caloric? We have seen that a very considerable quantity of heat may be excited by the friction of two metallic surfaces, and given off in a constant stream, or flux, in all directions, without interruption or intermission, and without any signs of diminution or exhaustion. In reasoning on this subject, we must not forget that most remarkable circumstance, that the source of the heat generated by friction in these experiments appeared to be inexhaustible. It is hardly necessary to add, that anything which any insulated body or system of bodies can continue to furnish without limitation cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in these experiments, except it be MOTION."
In style, Professor Tyndall's work is remarkably clear, spirited, and vigorous, and many of its pages are eloquent with the beautiful enthusiasm and poetic spirit of its author. These attractions, combined with the comprehensiveness and unity of the discussion, the range and authenticity of the facts, and the delicacy, originality, and vividness of the experiments, render the work at once popular and profound. It is s classic upon the subject of which it treats.
My Days and Nights on the Battle-Field. A Book for Boys. By "CARLETON." Boston: Ticknor and Fields.
The literature of the war has already reached the dimensions of a respectable library. The public mind at the instant of the outbreak felt an assurance that it was to be one of the memorable epochs of mankind. However blinded to the significance of the previous conflicts in the forum and at the ballot-box, there was a sudden and universal instinct that their armed culmination was a world-era. The event instantly assumed its true grandeur.
The previous discussions seemed local and limited. They were squabbles, we fancied, among ourselves, which did not touch the vitals of our system, and in which the world without had neither lot nor interest. Even when the fires of debate and division waxed hotter and hotter, and began to break out in violent eruptions in Congress, Kansas, throughout the South, and especially at Harper's Ferry, we still said, These are political conflicts, mob-violences, raids, abnormal eccentricities, which will pass quietly away, when the dynasty is changed, and the reins of power are fairly grasped by the successful rival.
Europe sends her doctors to witness our dissolution. They go South and see the mustering of arms and the intensity of purpose, and coming North find the whole community at their usual pursuits and pleasures, regarding the controversy as a mere political breeze, and the results in which it is beginning to issue as but the waves that ever for a short season roll fiercely after the storm.