19U] 



on Fluid Motions 



40 



narrow places, which determines the character of the flow in a large 

 variety., of cases. They are held back, as Fronde pointed ont, by 

 friction acting from the walls ; but, on the other hand, when they 

 lag they are pulled forward by layers farther in which still retain 

 their velocity. If the latter prevail, the motion in the end may not 

 be very different from Avhat would occur ini the absence of friction ; 

 otherwise an entirely altered motion may ensue. The situation as 

 regards the rest of the fluid is much easier when the layers upon 

 which the friction tells most are allowed to escape. This happens in 

 instruments of the injector class, but I have sometimes wondered 

 whether full advantage is taken of it. The long gradually expanding 

 cones are overdone, perhaps, and the friction which they entail must 

 have a bad effect. 



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Similar considerations enter when we discuss the passage of a 

 solid body through a large mass of fluid otherwise at lest, as in the 

 case of an airship or submarine boat. I say a submarine, because 

 when a ship moves upon the surface of the water the formation 

 of waves constitutes a complication, and one of great importance 

 when the speed is high. In order that the water in its relative 

 motion may close in properly behind, the after-part of the ship must 

 be suitably shaped, fine lines being more necessary at the stern than 

 at the bow, as fish found out before men inteiested themselves in the 

 problem. In a well-designed ship the whole resistance (apart from 

 wave-making) may be ascribed to shin friction, of the same nature as 

 that which is encountered when the ship is replaced by a thin plane 

 moving edgeways. 



At the other extreme we may consider the motion of a thin disk 

 or blade flatwavs throuii'ii the water. Here the actual motion differs 



