XVI] OF STREAM-LINES 965 



wing, all with so much waste of energy*. In short the broad wing 

 is less efficient than the narrow; and on either side of our sheet 

 of paper we may cut out a portion whiph is useless and in the way : 

 for the same reasons we may. cut out the middle part of the tail, 

 which also is doing more harm than goodf. 



Hard as the problem is, and harder as it becomes, we may venture 

 on. In aeronautics, as in hydrodynamics, we try to determine the 

 resistances encountered by bodies of various shapes, moving through 

 various fluids at various speeds; and in so doing we learn the 

 enormous, the paramount importance of "stream-lining." There 

 would be no need for stream-lining in a "perfect fluid," but in air or 

 water it makes all the difference in the world. Stream-lining impHes 

 a shape round which the medium streams so smoothly that resistance 

 is at last practically nil; there only remains the slight "skin- 

 friction," which can be reduced or minimised in various ways. But 

 the least imperfection of the stream-lining leads to whirls and 

 "pockets" of dead water or dead air, which mean large resistance 

 and waste of energy. The converse and more general problem soon 

 emerges, of how in natural objects stream-lining comes to be; and 

 whether or no the more or l^ss stream-lined shape tends to be 

 impressed on a deformable or plastic body by its own steady motion 

 through a fluid J. The principle of least action, the "loi de repos," 

 is enough to suggest that the stream will tend to impress its stream- 

 lines on the plastic body, causing it to yield or "give," until it ends 

 by ofl'ering a minimum of resistance; and experiment goes some 

 way to support the hypothesis. A bubble of mercury, poised in 

 a tube through which air is blown, assumes a stream-lined shape, 

 in so far as the forces due to the moving current avail against the 



* This is why "slotting" so improves the broad wing of the crow. See on this 

 and other matters, R. R. Graham's papers on Safety devices in the wings of 

 birds, in British Birds, xxiv, 1930. 



t Pettigrew shewed long ago {Tr. R.S.E. xxvi, p. 361, 1872) that the wing-area 

 (in insects) "is usually greatly in excess of what is absolutely required for flight," 

 and that the posterior or traihng edge could be largely trimmed away without the 

 power of flight being at all diminished. We see how in the swallow this trailing edge 

 is "trimmed away" till a bare minimum is left, and how (at least for a certain kind 

 of flight) the wing is thereby greatly improved. 



X Cf. Enoch Farrer, The shape assumed by a deformable body immersed in 

 a moving fluid, Journ. Franklin Inst. 1921, pp. 737-756; also Vaughan Cornish, 

 on Waves of Sand and Snow. 



