Page:Popular Science Monthly Volume 43.djvu/514

498 rise higher and become moro acuminate. Rankine has investigated the limiting forms which waves assume before breaking, and has concluded that in the steepest possible oscillatory waves of the irrotational kind, the crests become curved at the vertex in such a manner that a section of the crest by the plane of motion presents two branches of a curve which meet at an angle of ninety degrees.

After the prolonged action of the wind, when the crests of the waves rise to a considerable height and become sharper and sharper, the passage of the air over them with high velocity tends to impart its velocity to them. Owing to the inertia of the lower masses of water, the imparting of this velocity is resisted. The paths of the particles become distorted, as shown in Fig. 3, the front of each wave gradually becomes steeper than the back, and the crests seem to advance faster than the troughs, until at length the front of the wave curls over and breaks, as shown in Fig. 4.

Large sea-waves seem to be the result of a building-up process carried on by the joint action of large and small waves. If, for any cause, there be one wave larger than those surrounding it, its size will be continually increased at the expense of the smaller ones. For these smaller waves, in passing over the tops of the larger, offer increased obstruction to the wind and cause the formation of cusps when the waves coincide. The delicate equilibrium incident to a cusped form is easily destroyed by the action of the wind, and the crests of the waves break into fragments which go to increase the volume of large waves, leaving the small ones yet smaller. Therefore, whatever influence prevents the breaking of waves acts also as an agency to prevent their increase in size. No fact of observation and no method of sound reasoning has yet led to the conclusion that the spreading of oil on the surface of water agitated by waves can exercise any sensible effect in lessening the size or velocity of the waves themselves. It is in the breaking of the waves that the oil finds its field of action.

Having reviewed the structure of sea waves, the next step is to show why oil spreads over the surface of water. There is an attraction of one particle of water for another, and there is an attraction of one particle of oil for another, but there is a repulsion between a particle of water and a particle of oil. If we attempt to mix oil and water, the two liquids separate from each other of themselves, and in the act of separation sufficient force is brought into play to set in motion considerable masses of the fluids.

Imagine an individual particle of water within a mass of water.