be possible to attain. Suppose we take steam, the almost universal motive power of to-day, as an example, and put the inquiries, What ought we to get out of it and what do we get out of it? And when I am through, I think that many of my hearers, who have heretofore entertained the belief that steam-engineering was a field that had been so thoroughly worked up that but little remained to be accomplished in the direction of increasing the duty of our steam-motors, will be willing to acknowledge themselves mistaken.
To get at the practical duty of a steam-engine, we must begin with the source of the power, the steam-generator—popularly and most inappropriately called the steam-boiler; and, as the source and origin of the power generated in the boiler and directly traceable to the combustion of the fuel, it is evident that we must begin with that. Let us inquire, therefore, what power we ought to get from a perfect steam-engine burning pure coal, and then compare it with what we do get in the best steam-engine practice of to-day.
To understand the deductions I shall shortly make in getting at this comparison between theory and practice, I prefer to invite you to follow me through a few theoretical considerations, rather than ask you to accept the conclusions simply on my bare assertion.
It has long been known that a definite relation exists between the quantity of heat developed in a given operation and the quantity of mechanical force (manifested as work) that could be obtained from that heat. The absolute nature of this equivalency is tacitly recognized, though perhaps imperfectly comprehended in the practice of every branch of industry employing heat as a source of power; for it is this fact which establishes the dimensions of the steam-boiler, and the several proportions of the engine to do the work required of it. The steam-engine, in simple language, is simply an apparatus for turning heat into work; and it is, therefore, quite possible to express the value of a given quantity of the form of energy we call heat in terms of mechanical energy that we call “work”; and scientific investigation has established an admirable unit for this comparison in the “foot-pound”—that is, the force required to raise a pound weight to the height of one foot.
Now, to estimate the value of heat in terms of work, it was found necessary to determine the amount of mechanical force necessary to raise the sensible heat of one pound of water one degree in temperature. This amount has been carefully determined by several eminent savants, and has been given the name of the “mechanical equivalent of heat.” The value of this constant has been found to be 772 foot-pounds that is to say, the mechanical energy possessed by a body weighing one pound, after falling from a height of 772 feet, would, if it could all be converted into the form of energy we call heat, be exactly sufficient to raise the temperature of one pound of water 1° Fahr. (where the centigrade thermometer is employed, this constant
