on Waves in water supposed frictionless. 373 



ment. But no theoretical solution, and very little of experi- 

 mental investigation, can be referred to with respect to the eddy- 

 ings of the air blowing across the tops of the waves, to which, by 

 its giving rise to greater pressure on the posterior than on the 

 anterior slopes, the influence of the wind in sustaining and main- 

 taining waves is chiefly if not altogether due. 



My attention having been called three days ago, by Mr. Froude, 

 to Scott Russell's Report on Waves (British Association, York 5 

 1844), I find in it a remarkable illustration or indication of the 

 leading idea of the theory of the influence of wind on waves, 

 that the velocity of the wind must exceed that of the waves, in 

 the following statement : — " Let him [an observer studying the 

 surface of a sea or large lake, during tne successive stages of an 

 increasing wind, from a calm to a storm] begin his observations 

 " in a perfect calm, when the surface of the water is smooth and 

 " reflects like a mirror the images of surrounding objects. This 

 " appearance will not be affected by even a slight motion of the 

 " air, and a velocity of less than half a mile an hour (8J in. per 

 " sec.) does not sensibly disturb the smoothness of the reflecting 

 " surface. A gentle zephyr flitting along the surface from point 

 " to point, may be observed to destroy the perfection of the mir- 

 u ror for a moment, and on departing, the surface remains po- 

 " lished as before ; if the air have a velocity of about a mile an 

 " hour, the surface of the water becomes less capable of distinct 

 " reflexion, and on observing it in such a condition, it is to be 

 " noticed that the diminution of this reflecting power is owing 

 " to the presence of those minute corrugations of the superficial 

 " film which form waves of the third order. These corrugations 

 " produce on the surface of the water an effect very similar to 

 " the effect of those panes of glass which we see corrugated for 

 " the purpose of destroying their transparency, and these corru- 

 " gations at once prevent the eye from distinguishing forms at a 

 " considerable depth, and diminish the perfection of forms re- 

 " fleeted in the water. To fly-fishers this appearance is well 

 " known as diminishing the facility with which the fish see their 

 " captors. 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 inca- 

 " pable of travelling spontaneously to any considerable distance, 

 " except when under the continued action of the original disturb- 

 l< ing 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 an index of the presence of wind, and often as the forerunner 

 " of more. 



