552 VISCOSITY. [CHAP, xi 



on observing it in such a condition, it is to be noticed that the diminution of 

 this reflating power is owing to the presence of those minute corrugations of 

 the superficial film which form waves of the third order [capillary waves].... 

 This first stage of disturbance has this distinguishing circumstance, that the 

 phenomena on the surface cease almost simultaneously with the intermission 

 of the disturbing cause so that a spot which is sheltered from the direct action 

 of the wind remains smooth, the waves of the third order being incapable of 

 travelling spontaneously to any considerable distance, except when under the 

 continued action of the original disturbing force. This condition is the 

 indication of present force, not of that which is past. While it remains it 

 gives that deep blackness to the water which the sailor is accustomed to 

 regard as the index of the presence of wind, and often as the forerunner of 

 more. 



" The second condition of wave motion is to be observed when the velocity 

 of the wind acting on the smooth water has increased to two miles an hour. 

 Small waves then begin to rise uniformly over the whole surface of the water ; 

 these are waves of the second order, and cover the water with considerable 

 regularity. Capillary waves disappear from the ridges of these waves, but are 

 to be found sheltered in the hollows between them, and on the anterior slopes 

 of these waves. The regularity of the distribution of these secondary waves 

 over the surface is remarkable ; they begin with about an inch of amplitude, 

 and a couple of inches long ; they enlarge as the velocity or duration of the 

 wave increases ; by and by the coterminal waves unite ; the ridges increase, 

 and if the wind increase the waves become cusped, and are regular waves of 

 the second order [gravity waves]*. They continue enlarging their dimensions, 

 and the depth to which they produce the agitation increasing simultaneously 

 with their magnitude, the surface becomes extensively covered with waves of 

 nearly uniform magnitude." 



It will be seen that our theoretical investigations give con- 

 siderable insight into the incipient stages of wave-formation. No 

 sufficient explanation appears however to have been as yet given 

 of the origin of the regular processions of waves of greater length 

 which are so conspicuous a result of the continued action of wind 

 on a large expanse of water. 



304. The calming effect of oil on water waves appears to be 

 due to the variations of tension caused by the extensions and con- 

 tractions of the contaminated surface f. The surface-tension of 

 pure water is less than the sum of the tensions of the surfaces of 

 separation of oil and air, and oil and water, respectively, so that a 



* Scott Kussell's wave of the first order is the ' solitary wave ' (Art. 234). 



t Keynolds, "On the Effect of Oil in destroying Waves on the Surface of 

 Water," Brit. Ass. Rep., 1880; Aitken, "On the Effect of Oil on a Stormy Sea, 

 Proc. Eoy. Soc. Edin., t. xii., p. 56 (1883). 



