WORK, UNIT OF. The sum of the dynamical effect produced by a prime mover in a given time is known in books upon mechanism by the term of its unit of work ; but in order to bo able to compare the relative effective values of the various motors it is customary to refer them to the unit of work most generally considered to represent the normal conditions of an artificial prime mover. In such eases the amount of work is taken as being represented by a weight raised through a definite height in the time given ; and as horsepower was formerly the one most cummouly used for mechanical purposes, the custom has arisen of comparing the efficient power of machines in pro portion to the units of work they can perform, each of which units is equal to the unit of work performed by a horde supposed to raise a weight vertically. Writers upon physics are far from being unanimous as to the real value to bo assigned to the horse-power ; but in England it is generally considered to be equivalent to a weight of 33,000 lbs. raised one foot high in a minute; or to 550 lbs. raised 1 fuot in a second; and that unit of work is adopted as the term of comparison in the majority of cases when the powers of steam engines are com pared. In the case of the pumping engines of the Cornish mines, the unit of work is made to refer to the weight of water actually raised by the combustion of a hundred weight of coals, and the unit of work is technically known in that district by the term, " duty." Of course the amount of work performed by any prime mover must vary under the ever varying conditions of practice ; and especially when men, horses, or other animals, are employed must the effort they can exert depend upon individual constitution, and upon accidental circumstances. The units of work assigned to the respective motors mentioned below must, therefore, only be considered to represent a rude kind of average in each instance, and particular attention must be paid to the fact that there is for all of them a velocity of the point of application of the power, an effort, and a duration of that effort, which are the most favourable for the useful effect. The average unit
of work may often vary from I to I above or below the quantities cited, according to the age, or the health, of the animal motors, or to the climate in which they work. The table is extracted from Poncelet's ' Mdeanique The unit of work of steam engines, the horse-power, has long ceased to have any real meaning, for the real power, as ascertained by the dynamometer, is invariably in excess of the nominal horsepower of the engines. The common rule for calculating the nominal power of engines, whose pistons move at the velocities prescribed by Watt's rules, la however as follows : multiply the square of the diameter of the cylinder, in inches, by the velocity of the piston, in feet, and divide tho product by 6000; the product will be the number of nominal horses' power. As this formula does not include any term expressing the expansive power of the steam, it must evidently be unsatisfactory ; the real power maybe ascertained by means of the indicator as follows. Multiply the area of the piston by the residual pressure, after deducting for the friction and the loss of power in working the air pump (this is usually I Ij lbs.), and by the velocity of the piston in feet per minute ; the product divided by 33,000 will represent the real effective horse power applied to the main shaft. In both these cases, the unit of work of the borse-power is taken at 33,000 lbs. raised 1 foot high per minute. In Watt's time this amount of work could only be secured by the combustion of about 16 lbs. of coal per horse-power per hour ; at the present day, in ordinary condensing engines, only from 7 to 8 lbs. are required ; whilst in the best expansion-geared engines with surface condensation the same result is obtained by the combustion of 2,1be. of coal.