d to turn wheel 1 causes a
slower rotation of wheel 3, and a still slower rotation of wheel 4,
but as 4 rotates it winds up a chain and slowly raises _Q_. A very
complex machine is that seen in Figure 117.

[Illustration: FIG. 115.--A simple derrick for raising weights.]

[Illustration: FIG. 116.--A traveling crane.]

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 s