ART. 55. _Kinetic Energy._--Kinetic energy may be defined as energy of motion, and is the energy which a body possesses in consequence of its motion. A body in motion thus possesses kinetic energy, which it must impart to some other body before it can be brought to a state of rest. The body may be simply an atom, as a vortex atom, but if it be in motion, as all atoms are, then it must possess kinetic energy, which may be transferred to another atom by collision, or by some other method. As has already been pointed out in previous articles, kinetic and potential energy are complementary to one another, the sum-total of the two combined always remaining the same in any cycle of work, according to the principle of the conservation of energy. We get a good example of this oscillation from kinetic to potential, and _vice versâ_, in the planetary system. When the earth is farthest from the sun, its velocity, and consequently its kinetic energy, is at its lowest point; but there the potential energy is at its greatest. As the earth turns round in its orbit, however, and begins to approach the sun again, its potential energy decreases, while its kinetic energy increases with its increased velocity. So that by the time it has reached the nearest part of its orbit to the sun, its velocity, and consequently its kinetic energy, is at a maximum, while the potential energy is at a minimum. Then as the earth passes round its perihelion, the kinetic energy is used up in assisting the earth to overcome the attraction of the sun. Thus there is this oscillation from kinetic to potential, and from potential to kinetic, year in and year out, as the earth performs its cycle round its central body the sun. Professor Tait, in the work referred to in the previous Article, gives examples of kinetic forms of energy under the following heads-- 1st. Winds. 2nd. Currents of Water. 3rd. Hot Springs and Volcanoes. It can be readily seen that winds are a form of energy, as we have innumerable instances of the power and energy which they exert. Advantage is taken of that kinetic energy by means of windmills, in which the energy of the wind is imparted to the revolving sails, and thence to the machinery, various forms of mechanical work being the result, as, for example, the grinding of corn, or the pumping of water. The pressure or energy of winds has even been calculated, the following figures being examples-- VELOCITY IN MILES PER HOUR. FORCE IN LBS. PER SQ. FOOT 1 mile. .005 lb. per sq. foot. 5 " .123 " " " 10 " .496 " " " 15 " 1.11 " " " 20 " 1.98 " " " 30 " 4.5 " " " 40 " 7.9 " " " 50 " 12.5 " " "