Effect (Useful), in Mechanics; the measure of the real power of any machine, after deducting that portion which is lost or expended in overcoming the inertia and friction of the moving parts, and in giving them the required velocity, and every other source of loss. The greatest useful effect which a horse can produce, was estimated by Bolton and Watt at 33,000 lbs. raised one foot high per minute; and upon the introduction of the steam engine to perform the labour which was before performed by horses, their power was expressed by that of the number of horses which they were equal to; and from the convenience of this mode of expression, it has since been generally adopted to express the power of all kinds of machinery. It is at all times desirable to know the real effective power of any machine: when water is to be raised, the effect is readily computed, but in most other species of work it is difficult to estimate the resistance, and consequently whether the engine be really equal to its nominal power; in this case, Mr. Tredgold observes, the most convenient and simple mode of measuring the effect is by friction.

If the rim of a brake wheel of known diameter upon an engine shaft be pressed with a force producing a known degree of friction, which is exactly equal to the effect of the engine at its working speed, then it is clear that if the friction this pressure produces be ascertained, the power of the engine will be equal to the friction multiplied by the velocity of the rubbing surface. To apply this, let A B be a lever with a friction strap, that may be tightened upon the cylindrical surface of the shaft or wheel C, and let it be tightened by the screw at B (the lever being stopped by the stop at D), till the friction be equal to the power of the engine when all other work is thrown off; then while the engine is still in motion, add such a weight at E as retains the lever in a horizontal position. To calculate the power, multiply together the length of the lever F C in feet, me weight E in lbs. the number of revolutions of C per minute, and the number 6.2832; the result will be the number of pounds raised one foot per .minute, and divided by 33,000 it is the horses' power. Thus if a shaft make 20 revolutions per minute, and the length E C of the lever be 10 feet, and if it be found that a weight of 240 lbs. is sufficient to retain the lever in a horizontal position, then 6.2832 x 10 x 240 x 25=376992 lbs. raised one foot high, or nearly eleven and a half horses' power