So much for the practical effects of oil on broken water. Now let us proceed to examine the reasons why so small a quantity of oil can produce these effects. In order to understand the methods for opposing the violence of waves, it is essential that the phenomena which constitute wave motion be understood. It can be said with some degree of confidence that there is no instance in Nature of a perfectly quiescent surface of water. Air and water are both mediums of extreme mobility, and the individual molecules of both, and of all other substances, are continually in a state of motion, with different velocities, in paths different in direction and length. There is thus a continual interlacing of particles. When air covers water, some of the particles of air, in their excursions, strike the surface of the water, producing unequal pressures upon it, and giving rise to ripples which the vision is not acute enough to detect. If the original surface of the water were perfectly smooth, and if all parts of it continued equally exposed to an equal wind, waves could not be produced. But with the minute corrugations which are always present upon the smoothest water it is to be observed that it does not occur that water is all equally exposed to equal winds. The pressure of moving air upon the crests and posterior portions of the minute corrugations is greater than that on the hollows and anterior portions. There is thus a tendency to heap up the water at the places of greatest pressure, which is augmented by the rotational or vortex motion produced by the viscosity of the air. These actions produce new forms and inequalities, which, exposed to the wind, generate new modifications of its force and give rise to further deviations from the primitive condition of the fluid. Imagine an isolated example in which the water has been suddenly heaped up by a gust of wind. The action of gravity causes the particles of water in the heap to push forward the particles immediately in front of them out of their former place to another place farther on, and they repose in their new place at rest as before the original heaping up. Thus in succession volume after volume continues to carry on a process of displacement which only ends with the exhaustion of the displacing force originally impressed and communicated from one to another successive mass of water. As the particles of water crowd upon one another in the act of going out of their old places into the new, the crowd forms a temporary heap visible on the surface of the water, and as each successive mass is displacing its successor there is always one such heap, and this heap travels apparently at that point where the process of displacement is going on; and although there may be only one crowd, yet it consists of always another and another set of migrating particles. This moving crowd constitutes a true wave. The velocity of the wave is the velocity with which the heap is seen to move. Its form is
Page:Popular Science Monthly Volume 43.djvu/512
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THE POPULAR SCIENCE MONTHLY.