Page:Popular Science Monthly Volume 43.djvu/518

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802
THE POPULAR SCIENCE MONTHLY.

When wind blows over water, all the air does not pass over the surface of the water. On account of the high degree of adhesion between air and water, a thin stratum of air remains in contact with the water, and it is the action of the internal friction or viscosity of air tending to draw this stratum along which causes the tractive effect of wind on water.

When a film of oil is spread over the surface, this tractive force is not brought to bear on the surface of the water as long as the film remains unbroken, but acts upon the surface of the film, whose particles, being entirely separate from the particles of water, do not share their motion. The surface of the water is thus shielded from the action of the wind in the same manner as if a skin of India rubber were spread over it, and the only action of the wind in such a case is to move the film over the surface of the water.

It is calculated that a wind moving at the rate of twenty-five miles per hour or one hundred and twelve centimetres per second, relatively to the surface of the water, exercises a tractive effect of about two thousandths of a gramme upon each square centimetre of surface; and, when we consider that this force is brought to bear upon a system of particles moving in their orbits, in the direction in which the wind blows, with a speed of about eighty centimetres per second, it will be apparent that the interposition of a film of oil between the air and water must have a powerful effect in preventing breaking crests.

Observation has shown that, in the generation of oscillatory waves, ripples or capillary waves are first formed, and that it is to the union of conterminal ripples and to their more abundant formation with the increased force of the wind that the growth of waves is due. The existence of a certain definite tension, equal to ·08235 gramme per lineal centimetre, at the common surface of air and water has been pointed out. The water surface under this tension is in perfect equilibrium.

When wind blows over the surface of a body of water, the tangential force which the air, in virtue of its viscosity, exerts on the surface of the water, is of different degrees of intensity at different places, owing to the minute corrugations which are always present on the surface of a body of water, and to the eddying motion of the air. At the places where the tangential force is greatest, the surface film of water is drawn along and the portions of the surface immediately in front of them, destroying their surface tension or energy of position, and, by laying bare new surface in places from which they are moved, generating a like amount of surface tension. Through this action heaps or ripples are formed, and surface tension is being constantly generated and destroyed. The formation of ripples takes place on waves already in exist-