(175.)

In expressing the duty of engines, it would have been desirable that the duty of the boiler should have been separated from that of the engine.

The duty of a boiler is estimated by the volume of water evaporated by a given quantity of fuel, independently of the time which such evaporation may take. The duty, therefore, will be expressed by the number of cubic feet of water evaporated, divided by the number of bushels of coal necessary for that evaporation, supposing the bushel of coal to be the unit of fuel. It will be observed that the duty of an engine or boiler is entirely distinct from, and independent of, its power. One boiler may be greater than another in power to any extent, while it may be equal to or less than it in duty. A bushel of coals may evaporate the same number of cubic feet of water under two boilers, but may take twice as great a time to produce such evaporation under one than under the other. In such a case the power of one boiler will be double that of the other, while their duty will be the same.

In like manner, a bushel of coals consumed in working two engines may produce the same useful effect, but it may produce that useful effect in the one in half the time it takes to produce it in the other. In that case the duty of the engines will be the same, but the power of the one will be double that of the other. [Pg295]

In fine, power has reference to time,—duty, to fuel. The more rapidly the engine produces its mechanical effect, the greater its power will be, whatever may be the fuel consumed in working it. And, on the other hand, the greater the useful effect produced by a given weight of fuel, the greater will be the duty, however long the time may be which the fuel may take to produce the useful effect.