Page:Popular Science Monthly Volume 43.djvu/513

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WHY A FILM OF OIL CAN CALM THE SEA.

the form of the heap. Its length is the distance from crest to crest, and its height is the distance from the crest to the surface of the water before the disturbance.

The motions of the individual particles of water are different from the motion of translation which the wave has. Consider a PSM V43 D513 Direction of oil propagation on waves.jpgFig. 1. particle in a mass of water about to be traversed by a wave form. The action of gravity on the heap behind it tends to press it forward, where it is confronted by a solid wall of water. Under the action of these two opposite forces the particle is driven upward and forward until the particles which have displaced it have made room for themselves; then it sinks, and finally comes to rest a little in advance of the place from which it started. The motion of migration of each individual particle is thus in a closed orbit. The propagation of the wave is the advancement of a mere form. The actual translation of water in the propagation of unbroken waves is small. The motion of each particle takes place in a vertical plane parallel to the direction of propagation of the wave. The path of orbit described by each particle is approximately elliptic, and in water of nearly uniform depth the longer axis of the elliptic orbit is horizontal and the shorter vertical. When at the top of its path, the particle moves forward as regards the direction of propagation; when at the bottom, backward, as shown by the PSM V43 D513 Initial circulation of wind driven wave in deep water.jpgFig. 2. curved arrows in Fig. 1. The straight arrow denotes the direction of propagation. The particles at the surface describe the largest orbits. The extent of the motion horizontally and vertically diminishes with the depth below the surface. A particle in contact with the bottom of water of moderate depth moves backward and forward in a horizontal straight line, as at D. On the ocean, where the water is deep as compared with the length of a wave, the paths of the particles are nearly circular, and the motion is insensible at great depths.

When waves are first raised at sea their crests are smooth and rounded, as represented in Fig. 2. As the wind freshens the crests