The Work•Done by Steam-engines.—This is estimated in two ways—as horsepower, and as duty, and the first expression includes two things—nominal and indicated horse power. Thirty-three thousand foot-pounds of work done per minute is called one horse power, this being considered by Watt as the maximum force which a strong horse could exert. The nominal horse-power of an engine has long ceased to be any expression of the actual power it exerts; it is only used as a kind of commercial standard (a very defi cient one) for the sale and purchase of engines, and is generally made to depend entirely on the diameter of the cylinder.
The indicated horsepower is the most useful measure we have of the work done by an engine. It expresses, however, the total work done by the steam on the piston, and does not show at all what proportion of that work has to be expended in overcoming the friction of the engine itself. It is ascertained by the use of a little machine called an " indicator," devised by Watt, and since his time greatly improved, especially by Mr. Richards.
By taking the mean pressure per square inch on the piston throughout the stroke (deduced from the indicator card), and multiplying it by the area of the piston, and by the number of feet passed through by it in a minute, we should find the number of foot pounds of work done by the engine per minute; and this, divided by 33,000, would give the indicated horse-power.
" Duty" is a measure of power mused only for pumping engines, and differs from horse-power in being entirely independent of time. It is the number of foot-pounds of
net work resulting from the consumption of a given quantity of coal, usually either a bushel of 9-1 lbs. or a hundredweight. At the beginning of this century the maximum duty that had been attained by any Cornish engine was 20 millions of foot-pounds per cwt, of coal, but six times that duty has since been occasionally obtained. In these engines, it is the actual net work done which is taken into account; the duty would be 20 or 25 per cent greater if the total load on the steam-piston had been considered instead.
For engines whose power can only be measured by the indicator, the standard of economy is the number of lbs. of fuel used per hour per indicated hove-power. In fac tories where "dross" is used as fuel, with horizontal engines and Cornish boilers, and where no means are taken to insure economy. we have known 15 to 20 lbs of fuel burned per indicated horse-power per hour. In marine engines and other cases where the best coal only is used, and where high pressures, surface condensation, and compound cylin ders are employed, the consumption of fuel is often as low as 2 pounds.
The theory of a "perfect beat-engine," which should return in mechanical work (see FoncE) the greatest possible amount of the heat supplied to it, is considered under