Extinction of Waves at the Entrance of Harbours. 147 



To carry this simple principle out for practical purposes, 

 we have now to come to other considerations. Let us 

 suppose the apparent height of a wave, as seen from a boat 

 in the bottom of the trough, to be twenty feet ; now, as 

 half of this elevation belongs to the trough, the actual 

 height of the wave above the mean level of the sea is only 

 ten feet; supposing the sides to be inclined at an angle of 

 forty-five degrees, the area of a transverse section of that 

 wave will be one hundred square feet ; and that area, 

 multiplied by the length, will give the solid bulk of all the 

 floats required to extinguish it. 



A number of these elements in a double or multiplied row 

 would probably be more convenient than a few large ones, 

 and the construction, not being continuous, might be carried 

 out progressively until sufficiently effective. 



One advantage in contradistinction to a sea stone wall 

 would be, that the depth of the water would make very 

 little difference in the expense — being a question of length 

 of tether only, 



Among the obvious objections to the scheme must be 

 mentioned the variations of tide — the length of tether 

 suiting one tide not being suitable for another. In reference 

 to this point, it would seem more important to provide 

 against waves at high tides than at low ones ; but in any 

 case this is only a question of multiplying the elements, or 

 altering the form of the floats, probably by lengthening in 

 the perpendicular direction. 



However, the scheme, such as it is, is at the service of the 

 public, and I cannot help believing that the time will come 

 when some good will arise out of it. 



L 2 



