308 SECTION MECHANICS. 



each other, and the pipe experiences, on the whole, no endway push. 

 The exact modus operandi by which the particles adjust themselves 

 mutually to the change of speed is somewhat complicated, but that 

 the fact must be as described is I think obvious, and the fact is that 

 every contracted stream in the flow of an infinite ocean of fluid is a 

 region of increased speed and decreased pressure, and every enlarged 

 current is a region of decreased speed and increased pressure, and these 

 changes of pressure, if the fluid be frictionless, can produce, on the 

 whole, no endways push on the body whose presence causes the 

 changes of speed and pressure. 



Now I think I may proceed to treat of the case of a fish moving 

 under water, or, rather, of the streams, so to speak, in the infinite 

 ocean flowing past a stationary fish, and show how the alterations 

 of speed and pressure which they experience would, but for friction, 

 result in an equilibrium of endways force on the fish. At the nose of 

 the fish, when the streams are being deflected outwards to get past 

 him, forming lines convex towards his nose, their aggregate centrifugal 

 force piles up an excess of pressure, constituting what I have called a 

 region of increased pressure, which must also be a region of diminished 

 speed and of enlargement of sectional area in the streams contiguous 

 to the nose, and there is thus established an increase of sternward pres- 

 sure on the nose, which however we shall presently see is counteracted 

 by an equivalent increased pressure on the tail. Along the fish's middle 

 body, where the streams, which have been put into outward motion by 

 the nose, are losing that motion, and by a reversal of curvature are being 

 deflected inwards into convergence, the modification of pressure just 

 described is reversed, establishing in the contiguous streams a region of 

 diminished sectional area, of increased speed, and of diminished pres- 

 sure, which acting partly on the widening forward half of the middle body, 

 partly on its narrowing after end, results in an equilibrium of endways 

 force : while at the tail where the converging streams are again being 

 deflected into purely parallel sternward lines, an excess of pressure, 

 depending on the same conditions as that experienced by the nose, and 

 exactly equivalent to it, becomes established. Thus, in the frictionless 

 fluid, equilibrium of endway pressure on the fish would on the whole 

 subsist, and when once put in motion with a steady speed the fish 

 would experience no resistance. 



