Page:Popular Science Monthly Volume 58.djvu/476

468 is through the action of its attribute forces upon our senses; it is indeed possible that matter only exists through that quality which makes it the residence of the physical forces; it is extremely probable that all natural forces affect all matter and originate in matter.

There are just three corollaries to the general 'Law of Substance' the Law of Persistence of all Existences; these are:

 1. The Law of the Persistence of Matter per se. 2. The Law of Persistence of Force as an Attribute of Matter. 3. The Law of Persistence of Energy, whether as affecting a mass of matter or in process of transfer or of transformation; affecting varying quantities and kinds of matter; passing from one quantity of matter to another; changing, in inverse direction, the quantity of matter affected and the velocity-component of the energy; the product of mass and mean velocity-square remaining constant for the whole universe.

The distinction between force and energy was not, in earlier times, very exactly observed; but it is easy to perceive in the context to the enunciation of either corollary to the fundamental law the fact that writers usually well understood the principle which they sought to state. It had, by Faraday's time, come to be well understood by many scientific men that matter is persistent, that its characteristic forces cling to it persistently and that energy is the product of forces and motion, and is consequent upon inertia.

The writer took occasion, in a paper read before the American Society of Civil Engineers (December 9, 1873), criticizing Professor Tait's 'Sketch of Thermodynamics/ his assignment to Sir Humphry Davy of a prior place and his depreciation of the work of Mayer, to show that Rumford is entitled to a larger credit than is ordinarily assigned him even by those who admit his first appearance in this line of investigation at the close of the eighteenth century. It is easy to show that, not only was Rumford the first to exhibit by experimental research the fact of the equivalence of thermal and dynamic energy, but that he was the first to establish with some degree of approximation their quantivalence. In fact, he secured data giving a much closer determination of the 'mechanical equivalent of heat' than did Joule, or any other investigator of later years up to the middle of the century; at which date, while an approximate value had been hit upon, so great was the variety of constants published that the real value was still exceedingly uncertain. Professor Tait, however, was the first to call attention to the fact that Rumford actually gave data sufficient to afford a basis for computation of the equivalent, but he made the resultant figure 940 foot-pounds, assuming the horse-power at 33,000 foot-pounds per minute, and failing to note the fact that the engineer's 'horse-power' is considerably larger than the power of the average horse.

Taking the generally accepted and fair mean value for the power of