Abstract :
The speed of an engine may be expressed in revolutions per minute, or in terms of the linear velocity of the crank-pin in feet per minute. Revolutions per minute is rather more indefinite than crank-pin velocity in feet per minute, because feet per minute is accepted as not simply a measurement for a minute of time, whereas revolutions per minute more frequently is. Revolutions per minute may mean half the actual number of revolutions completed in two minutes or twice the number of revolutions completed in one-half minute, or it may mean the momentary rate of completing a revolution without any implied time. As there is a possibility of a lack of agreement on the implication in the term revolutions per minute, I should prefer to define the engine-speed in terms of crank-pin velocity in feet per minute, which can be expressed momentarily for any period of time, and does not imply any particular time. This velocity so expressed is the integral of the accelerating forces with respect to time, and in fact may be so defined according to the laws of mechanics.
