This equivalence and transformation prevail in all forms of energy, whether it be mechanical energy, thermal or heat energy, or electrical energy. ART. 54. _Potential Energy._--Energy has been divided into two classes, which are termed respectively Potential Energy and Kinetic Energy. We will look at the former first. _Potential Energy_ may be briefly defined as energy of position. Thus if we lift a body from the ground, the energy which has been imparted to it is energy of position, or potential energy. A glacier high up the mountain possesses potential energy, because of its position. By the mere fact that it is situated high up the mountain, it has a capacity for doing work by its descent, and if that descent be very sudden, the work done will be destructive work, as it may sweep away all houses and villages in its sudden descent. Thus, by the mere fact of its elevation, it possesses a power of doing work, which it has lost when it has descended. Again, work done in winding up the spring of a clock is stored up in the form of potential energy, and gradually runs out in the form of motion or kinetic energy. Potential energy is really the complementary principle of kinetic energy. That is to say, the amount of potential energy lost by any body, is equal to the amount of kinetic energy gained by the other body, to which the energy has been transferred. In the case of a body falling, as the potential energy diminishes, the kinetic energy increases, but the total amount of the two combined always remains the same. This is well illustrated in the case of a swinging pendulum. When a pendulum is at the highest point of its swing, its velocity or kinetic energy is zero, but at that point its potential energy is greatest. As it descends, the potential energy decreases, but the kinetic energy increases. When the pendulum is at the lowest point its energy is wholly kinetic, the potential energy being zero at that point, while it has sufficient kinetic energy to raise it to the highest level again. Throughout the cycle of these operations, the sum-total of the two energies always remains the same. Professor Tait points out, in his _Recent Advances in Physical Science_, that the available sources of all potential energy may be divided into four classes-- 1st. Fuel. 2nd. Food of Animals. 3rd. Water-power. 4th. Tidal Water-power.