Page:Popular Science Monthly Volume 15.djvu/306

 will have a value of 772 &times; 1.8 $$=$$ 1,390 foot-pounds). Now, this much having been gained in fixing the principle of our calculations, let us go back to our steam-boiler, and to the coal we feed it with. It has been experimentally determined that, if the entire quantity of heat given off during the burning of one pound of pure coal could be applied without loss to heating water, it would suffice to raise the temperature of one pound of water 7,900° C.; or, what is the same thing, differently stated, it would be sufficient to raise the temperature of 7,900 pounds of water one degree. The possible mechanical duty of the “theoretically perfect” steam-engine is found by simply multiplying the quantity which expresses the thermal equivalent of coal by the quantity which expresses the mechanical equivalent of heat, and the result would be the true value of one pound of coal burned in the boiler in “foot-pounds.” Performing this simple arithmetical operation, we obtain (7,900 &times; 1,390 =) 10,980,000 foot-pounds; or, to put it more simply, suppose we convert these foot-pounds into horse-power, which we can do by another simple arithmetical operation of dividing them by 33,000, and we shall have as a result that one pound of pure coal, burned in the perfect boiler in one minute, would, if we could apply it with absolute economy to the performance of work, exert a force of ($10980000⁄33000$ =) 332 horse-power during one minute; or, if burned during an hour, then one sixtieth of 332, or 5.5 horse-power.

With a perfect boiler, therefore, we ought to get 5.5 horse-power per hour out of every pound of coal burned on the grate-bars. Now, let us inquire, What do we get in practice? Surely, you will say, our scientific mechanics and engineers have succeeded in getting a goodly percentage out of this possible figure; and the splendid engines, of massive construction, that work so beautifully as to excite our wonder and admiration at their smoothness and ease of their movements, must be very near perfection. Alas for the vanity of human expectations! Instead of getting 5.5 horse-power out of every pound of coal we burn in the boiler, the very best boiler and engine that have ever been constructed require two and a half pounds of coal to give out one horse-power: which means that, in spite of the vaunted progress of the mechanic arts in our times, the best engineering talent applied to the improvement of the steam-engine, from the time of James Watt down to Corliss, has only succeeded in making it yield a duty of 15 per cent. of what it ought to do, leaving an enormous margin of 85 per cent. for future improvements.

In the foregoing remarks I have, I fear, inadvertently been unjust to our engine-builders, for by far the greater portion of this 85 per cent. of wasted power is chargeable directly to the steam-boiler, and but a comparatively small proportion thereof to the engine. In considering the question of the duty of steam-motors, however, we must take the whole machine (engine and boiler), as a single apparatus. If our boiler-makers could do as well as our engine-builders—the two