both represent a moving force. The power derived from them is the same.

Kinetic Energy.—The energy of the falling water or stone is called Kinetic energy. In both cases the power developed came from within themselves and not from any exterior source.

The difference between Potential and Kinetic Energy is therefore that Potential Energy represents the capacity to do work, while Kinetic Energy is the actual performance of work.

Friction.—In every form of energy there is always something to detract from it or take away a portion of its full force, called friction. When a shaft turns, it rubs against the bearings, and more or less power is absorbed.

When a wheel travels over the ground friction is ever present. The dislodging of the stone required ten pounds of energy, but a thousand pounds was developed by the fall. The water rushing along its rocky bed has friction all along its path.

Resistance.—This friction is a resistance to the movement of a body, and is ever present. It is necessary to go back and examine the reason for this. As long as the stone was poised at the top of the precipice it had latent or potential energy, which might be termed power at rest. When it fell it had power in motion. In both cases gravity acted upon the stone, and in like manner on the water pouring over the falls.

Inertia.—Inertia or momentum is inherent in all things and represents the resistance of any body or matter, to change its condition of rest or standing still into motion, and is then called Inertia of Rest, or the resistance it offers to increase or decrease its speed when moving, and is then called Inertia of Motion.

Inertia or momentum is composed by the weight of the body and its speed and is measured by multiplying its weight by its speed.

The law is, that when a body is at rest it will remain at rest eternally, and when in motion it will continue in motion forever, unless acted on by some external force or resistance. An object lying on the ground has the frictional resistance of the earth to prevent its moving. When the object is flying through space it meets the air and has also the downward pull of gravity, which seek to bring it to rest.

These resisting forces are less in water, and still less in gases, and there is, therefore, a state of mobility in them which is not found in solids.

Internal and External Resistance.—All bodies are subject to internal, as well as external resistance. The stone on the cliff resisted the movement to push it over. Weight was the resisting internal force, but when the stone was moving through the air, the friction with the air created external resistance.

Energy Indestructible.—There is another thing which should be understood, and that is the absolute indestructibility of energy. Matter may be changed in form, or in the direction of its motion, by the change of kinetic into potential energy, or vice versa, but the sum total of the energy in the world is unalterable or constant.

The tremendous power developed by the stone when it plunged through space and struck the rocks below, developed a heat at its impact. Thus the moving force which was a motion in one direction was converted into another form of energy, heat. The expansion of the material exposed to the heat also represented energy.

When powder explodes and absolutely changes the form of the substance, its volume of expansion, if it should be retained within a vessel, would perform a certain amount of work, and the energy is thus transferred from one form to another without ceasing.

Wind Power.—Primitive man also saw and felt the winds. He noted its tremendous power, but he could not see how a force moving in one direction only could be utilized by him.

Rectilinear Motion.—This movement of the wind in one direction, like the water flowing along the bed of the river, is called rectilinear motion. It required invention to convert rectilinear into circular motion.

Oscillating Motion.—When he threshed his grain and winnowed it by shaking it to and fro, to rid it of the chaff, the idea of using the wind to produce an oscillating motion did not occur to him. After circular motion was produced, the crank was formed and thus the oscillating movement was brought about.

Movements in Nature.—All movements in nature are simple ones, of which the following are illustrations:

1. Rectilinear, which, as stated, means in a straight line.

2. Circular, like the motion of the earth on its axis, once every twenty-four hours.

3. Oscillatory, like a to and fro movement, the swaying branches of trees, or the swinging of a pendulum.

How Man Utilizes the Various Movements.—What man has done is to utilize the great natural forces in nature in such a way as to produce these movements at will, in either direction, with greater or less speed, at regular or irregular intervals, and at such amplitudes as are required to perform the necessary work.

Kinds of Potential Energy.—Now, materials have within themselves potential energy of various kinds. Thus, powder, if ignited, will burn, and in burning will expand, or explode, as we term it. This is true also of oils and gases. The expansion pressure produced from such substances depends on the speed at which they will burn, and in so confining the burning substances that a great pressure is produced.

The Power in Heat.—The pressure of all such substances against the confining medium depends on heat. Any gas which has 523 degrees of heat imparted to it will expand double its volume. If one cubic inch of water is converted into steam the latter will occupy one cubic foot of space under atmospheric pressure,—that is, it will expand over 1700 times.

Energy in Steam.—If the steam thus generated is now subjected to 523 degrees of heat additional, it will occupy over 3400 cubic inches of space. It will thus be seen why steam, gas, and gasoline engines are called heat engines, or heat motors.

Energy From the Sun.—Many attempts have been made to utilize the heat of the sun, to turn machinery, but the difficulty has been to secure sufficient heat, on the one hand, and on the other to properly cool down the heated gases, so that the various liquid and solid fuels are required to make the heat transformations.

Power From Water.—In the use of water two forms are available, one where the water is moving along or falling in a constant open stream; and the other where the flowing water is confined and where its flow can be regulated and controlled. The latter is more available for two reasons:

First: Economy in the use of water.

Second: Ability to control the speed or movement of the motor.

With running or falling streams a large surface is required, and the wheels turn slowly. Two well-recognized forms of wheels have been employed, one called the undershot, or breast wheel, shown in Fig. 1, and the other the overshot, illustrated in