be lifted 550 feet in one second, or 550 pounds one
foot in the same time, it would be designated as one horse-power. For
that reason it is called a foot pound. Instead of using the figure to
indicate the power exerted during one minute of time, the time is taken
for a minute, in all calculations, so that 550 multiplied by the number
of seconds, 60, in a minute, equals 33,000 foot pounds.

FOOT POUNDS.--The calculation of horse-power is in a large measure
arbitrary. It was determined in this way: Experiments show that the heat
expended in vaporizing 34 pounds of water per hour, develops a force
equal to 33,000 foot pounds; and since it takes about 4 pounds of coal
per hour to vaporize that amount of water, the heat developed by that
quantity of coal develops the same force as that exercised by an average
horse exerting his strength at ordinary work.

All power is expressed in foot pounds. Suppose a cannon ball of
sufficient weight and speed strikes an object. If the impact should
indicate 33,000 pounds it would not mean that the force employed was one
horse-power, but that many foot pounds.

If there should be 60 impacts of 550 pounds each within a minute, it
might be said that it would be equal to 1 horse-power, but the correct
way to express it would be foot pounds.

So in every calculation, where power is to be calculated, first find out
how many foot pounds are developed, and then use the unit of measure,
33,000, as the divisor to get the horse-power, if you wish to express it
in that way.

It must be understood, therefore, that horse-power is a simple unit of
work, whereas a foot pound is a compound unit formed of a foot paired
with the weight of a pound.

ENERGY.--Now _work_ and _energy_ are two different things. Work is the
overcoming of resistance of any kind, either by causing or changing
motion, or maintaining it against the action of some other force.

Energy, on the other hand, is the power of doing work. Falling water
possesses energy; so does a stone poised on the edge of a cliff. In the
case of water, it is called _kinetic_ energy; in the stone _potential_
energy. A pound of pressure against the stone will cause the latter, in
falling, to develop an enormous energy; so it will be seen that this
property resides, or is within the thing itself. It will be well to
remember these definitions.

HOW TO FIND OUT THE POWER DEVELOPED.--The measure of power produced by
an engine, or other source, is so in