ON NA VAL ARCHITECTURE. 309 



But water is not a frictionless but a frictional fluid. The conse- 

 quence of that circumstance is that the streams as they flow along the 

 side, do experience a great frictional drag, and pull the fish backwards 

 by dragging along its skin. It may seem absurd to say that so smooth 

 a surface as that of a fish can experience such a frictional drag, but it 

 is the fact ; indeed experiment shows that a slippery surface like that of 

 a fish, experiences more, not less, frictional resistance than a hard 

 smooth one ; and I may tell you that this friction on the surface not 

 only pulls the fish back, but like the friction of any other body also 

 disturbs that beautiful arrangement which constitutes the system of 

 stream line motions and forces. The mutual drag of the surface and 

 the water, extinguishes in frictional eddies some of the energy of the 

 contiguous streams, and prevents them from re-instating the pressure 

 at the stern of the fish. In the flow of water through pipes a great 

 amount of energy is lost in friction. Even in a straight pipe, as in a 

 contracted pipe or^ an enlarged pipe, there is a. loss of work by friction, 

 and that is exactly what happens in the case of a submerged fish. 

 The fish would experience no resistance in frictionless water, but it 

 does experience resistance from friction in actual water. 



Now to go to the case of a ship moving on the surface. First I will 

 suppose the ship to be cut off at the level of the water, and I will sup- 

 pose the surface of the water to be held down by a sheet of rigid 

 frictionless ice which the ship will touch, but with no pressure ; that 

 ice-sheet, offering a reaction to the surface of the water equivalent to 

 that of an infinitely extended ocean, would enable the streams I have 

 just described in the ocean of a frictionless fluid, to flow past the ship 

 just as they would past a fish in a frictionless ocean at a great depth 

 beneath, that is to say, without tending to push her stern-ways. 



The ice surface would in effect constitute the remainder of the 

 ocean ; but there would be impressed upon the ice considerable force 

 by the differentiated pressures induced in the stream lines as they flow 

 past. At the head end of the ship, where there is an excess of pres- 

 sure from the streams being retarded, the ice would be ptfshed upwards ; 

 along the sides, where the water is accelerated and there is a diminu- 

 tion of pressure, the ice would be pulled downwards, and at the stern 

 again the ice would be pushed upwards ; there would be a stress 

 put upon it. Now, if we were to remove the ice, the water which ha 



