49 8 THE POPULAR SCIENCE MONTHLY. 



rise higher and become more acuminate. Rankine has investi- 

 gated 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 re- 

 3. sisted. The paths of the particles become dis- 



torted, 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 for- 

 mation of cusps when the waves coincide. The delicate equilib- 

 rium 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 ob- 

 servation and no method of sound reasoning 

 has yet led to the conclusion that the spread- 

 ing of oil on the surface of water agitated by F. 4. 

 waves can exercise any sensible effect in less- 

 ening 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. 



