32 HARBOUR CONSTRUCTION. 



" breaks." So soon, however, as the shoal has been passed, the 

 mass of the wave under-runs the broken crest, and the wave 

 proceeds as one of reduced volume, the amount of reduction 

 varying according to the depth of water over the shoal and the 

 height of the wave traversing it. 



Wind-crests and the irregularity which is often present in 

 the run of waves during the height of a gale tend, in some 

 degree, to aerate the waves. This may account for the well- 

 known fact that more damage is often done to sea-works during 

 the period in which a gale is subsiding than during its height. 

 The waves appear to become more regular, and in a manner to 

 steady themselves, as the force of the wind diminishes, and they 

 are thus better able to throw their full weight against any 

 opposing structure. 



I am indebted for much of what follows respecting the velocity 

 of waves, and indeed for a great deal of information on other 

 points, to the admirable treatise (to which I have already 

 referred) on " Tides and Waves," by Sir G. B. Airy. 



Velocity of Waves. When the length of a wave (crest to 

 crest) is not greater than the depth of the water, the velocity of 

 such wave depends (sensibly) only on its length, and is propor- 

 tional to the square root of its length. 



When the length of a wave is very much greater than the 

 depth of the water, or, in other words, when the water is very 

 shallow compared with the length of the wave, the velocity of 

 such wave depends (sensibly) only on the depth, and is pro- 

 portional to the square root of the depth. It is, in fact, the same 

 as the velocity which a free body would acquire by falling from 

 rest, under the action of gravity, through a height equal to half 

 the depth of the water. The formula for accelerated motion is 

 therefore applicable, viz. 



V = V^S = 8-02-v/g" 



V = Velocity of wave in feet per second. 

 g = Unit of force of gravity, or the velocity which a falling 

 body attains, in vacuum, at the end of the first second 

 = 3217 feet per second. 

 S = Space in feet fallen through, or, in this case, half the 



depth of the water. 



Long waves in deep water have therefore the greatest 

 velocity, regardless of their height. 



