=Wave-motion.=[151]—The most common waves, and from the present point of view the most important, are those generated by winds. During the passage of a wave, each particle affected by it rises and falls, and moves forward and backward describing an orbit in a vertical plane. If the passing wave is a swell, the orbit of the particle is closed and is either a circle or an ellipse; but in the case of a wind-wave the orbit is not closed. In such a wave two things move forward, the undulation and the water. The velocity of the undulation is relatively rapid; that of the water, slow and rhythmic. On the crest of the wind-wave each particle of water moves forward, and in the trough it moves less rapidly backward, and the excess of the forward movement over the backward gives it a slight residual advance. This residual advance is the initiatory element of current. By virtue of it, the upper layer of water is carried forward with reference to the layer below, in the direction toward which the wind blows. The waves of any considerable or long-continued wind, therefore, generate a current tending in the same direction as the wind. The agitation of which waves are the superficial manifestation is not restricted to the surface, but is propagated indefinitely downward. Near the surface the amount of motion diminishes rapidly with increasing depth (Fig. 299), but the rate of diminution itself diminishes, and there seems no theoretic reason for assigning any definite limit to the downward propagation of the oscillation. [Illustration: +Fig.+ 299.—Figure illustrating the decrease in the amount of wave-movement with increase of depth. (Fenneman.)] At the surface, the radius of the circular orbit which a particle of water in a wave tends to describe is half the height of the wave. At a depth equal to one wave-length, the radius of the circle described by a particle is ¹⁄₅₃₅ as great as at the surface, and at a depth equal to two wave-lengths, ¹⁄₃₀₀₀₀₀. If the height of a wave be 43 feet, the radius of the circle described by a surface particle is 21½ feet. If the length of the wave be 300 feet, the radius of a particle at a depth of 300 feet is only about ⁴⁄₁₀ of an inch, and at 600 feet ¹⁄₁₂₀₀ of an inch.[152] These figures make it clear that effective agitation of the water does not extend to great depths.