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