167.  Measurement of Work.  In Section 150, we learned that the amount of work done depends upon the force exerted, and the distance covered, or that W = force x distance.  A man who raises 5 pounds a height of 5 feet does far more work than a man who raises 5 ounces a height of 5 inches, but the product of force by distance is 25 in each case.  There is difficulty because we have not selected an arbitrary unit of work.  The unit of work chosen and in use in practical affairs is the foot pound, and is defined as the work done when a force of 1 pound acts through a distance of 1 foot.  A man who moves 8 pounds through 6 feet does 48 foot pounds of work, while a man who moves 8 ounces (1/2 pound) through 6 inches (1/2 foot) does only one fourth of a foot pound of work.

[Illustration:  FIG. 117.—­A farm engine putting in a crop.]

168.  The Power or the Speed with which Work is Done.  A man can load a wagon more quickly than a growing boy.  The work done by the one is equal to the work done by the other, but the man is more powerful, because the time required for a given task is very important.  An engine which hoists a 50-pound weight in 1 second is much more powerful than a man who requires 50 seconds for the same task; hence in estimating the value of a working agent, whether animal or mechanical, we must consider not only the work done, but the speed with which it is done.

The rate at which a machine is able to accomplish a unit of work is called power, and the unit of power customarily used is the horse power.  Any power which can do 550 foot pounds of work per second is said to be one horse power (H.P.).  This unit was chosen by James Watt, the inventor of a steam engine, when he was in need of a unit with which to compare the new source of power, the engine, with his old source of power, the horse.  Although called a horse power it is greater than the power of an average horse.

An ordinary man can do one sixth of a horse power.  The average locomotive of a railroad has more than 500 H.P., while the engines of an ocean liner may have as high as 70,000 H.P.

169.  Waste Work and Efficient Work.  In our study of machines we omitted a factor which in practical cases cannot be ignored, namely, friction.  No surface can be made perfectly smooth, and when a barrel rolls over an incline, or a rope passes over a pulley, or a cogwheel turns its neighbor, there is rubbing and slipping and sliding.  Motion is thus hindered, and the effective value of the acting force is lessened.  In order to secure the desired result it is necessary to apply a force in excess of that calculated.  This extra force, which must be supplied if friction is to be counteracted, is in reality waste work.

If the force required by a machine is 150 pounds, while that calculated as necessary is 100 pounds, the loss due to friction is 50 pounds, and the machine, instead of being thoroughly efficient, is only two thirds efficient.