8 HARBOUR CONSTRUCTION. 



I cannot do better than here quote what Abercromby says 

 on this subject in his excellent work before referred to. On 

 p. 202, he states as follows : 



" The velocity of wind is a real quantity, which is perhaps capable 

 of measurement in the abstract, though we are at present far from 

 being able to gauge it accurately. But it is quite certain that there 

 is no such thing as an absolute force which corresponds to a given 

 velocity. 



" According to the theory of stream-lines, when even an inelastic 

 fluid meets an obstacle, if the angles of the obstruction do not break 

 the continuity of the fluid, so as to form eddies or vortices, the same 

 amount of pressure which is imposed on the body by the first deflec- 

 tion of the fluid is given back again as the stream-lines of the fluid 

 close np behind the obstruction. For instance, if a ship is lying at 

 anchor in a current, the same amount of strain which the current 

 causes on her cable when forced asunder by the bows, is given back 

 when the current closes in behind her; so that the total pressure 

 which she experiences is only that due to the friction of the water on 

 her skin. This is, of course, on the supposition that her lines are so 

 easy that they do not break the stream-lines so as to form little 

 eddies or vortices. 



" Now, the same thing holds with wind. If we put up two square 

 plates of different sizes, face to the wind, the pressure on each is not 

 proportional to the area, while in light breezes neither will record 

 anything. The reason is that, in light wind, a thin mobile fluid like 

 air can glide round even the sharp angles of a square without form- 

 ing eddies, and as there is no vacuum formed behind the plate, there 

 is no pressure recorded. In higher winds the stream-lines are 

 broken, and every shape and every sized plate of the same shape 

 form a different series of eddies round the rim of the obstacle. Then 

 the amount of rarefication behind the various plates is neither 

 identical nor proportional, and therefore every shape and size of 

 anemometer indicate discordantly at every different velocity. 



" From all this, it follows that, though we might say that the 

 pressure on a board one foot square was twenty pounds, and might 

 compare this force with that on another board of the same size and 

 mounting, we should not be justified in saying that the force of the 

 wind was twenty pounds per square foot in the abstract, because a 

 board ten feet square, even if of the same shape, would have given a 

 different number." 



As further bearing on this subject, I quote the following 

 from Hurst's "Handbook for Surveyors" (14th edit., p. 140): 



