Note 88 — Chapter I

The term horse-power is employed to express the magnitude or capacity and power of an engine. It originated with James Watt from the actual measure of the work which a horse could perform, in raising 33,000 lbs. one foot high per minute; but as any such measure must, in the nature of things, be vague and fluctuating, it was replaced by what is now known as “nominal horse-power,” a mode of measurement based mainly upon the area of the cylinder, the number of strokes per minute and the pressure. But this method is far from showing the actual horse-power, as some modern engines will give an effective power three, four, and even six times greater than the nominal; it serves, however, as a commercial unit of capacity or power of performance and regulates the price to be paid for an engine. But it is much to be regretted that nominal power is not yet estimated by an uniform standard, as different rules are still applied to condensing and non-condensing engines, and these vary in different places.