All About Engines

Power and Its Measurement 3*3 all gases and vapours can be used to convert heat into mechanical work. Liquids are useless, because the alteration of volume with change of temperature is so very small. And of all vapours that of water is for many reasons the most convenient, as it is the most widely distributed and plentiful in the world. Let us glance now at that much-abused word “power.” In one sense it merely denotes general ability or strength, as in speaking of a powerful man. But in reference to engines it signifies “ rate of doing work.” A thousand foot-pounds may be done in an hour, or it may be done in a minute, and the one is sixty times greater than the other. To measure the rate at which work is done we must have a unit, and that chosen is 33,000 ft.-lb. per minute. It is called a horse-power, though it would require a pretty strong horse to raise 33,000 lb. 1 ft. from the ground, or i lb. 33,000 feet from the ground in sixty seconds ! A given amount of work can be done slowly or it can be done quickly, but it is done all the same. But if the time in which it is to be done is specified, then what we have in mind is not so much the work to be done as the rate of doing it. We are thinking of power, and the units of measurement are not foot- pounds, but foot-pounds per minute. Dividing this quantity by 33,000 gives the result in horse-power. How is the horse-power of an engine measured ? Consider the piston in the cylinder with the steam pressing on it. The pressure in pounds per square inch multiplied by the number of square inches of