964 ON FOKM AND MECHANICAL EFFICIENCY [ch. 



flapping flight ; how the sea-gull does not need them, for his load is 

 lighter and his wings move slowly. There is never a discovery 

 made in the theory of aerodynamics but we find it adopted already 

 by Nature, and exemplified in the construction of the wing*. 



We may illustrate some few of the principles ijivolved in the con- 

 struction of the bird's wing with a half-sheet of paper, whose laws, 

 as it planes or glides downwards, Clerk Maxwell explained many 

 years ago|. 



To improve this first and roughest of models, we see that its 

 leading edge had better be as long as possible, and that sharp 



Fig. 458. A diagrammatic bird. 



corners are bound to cause disturbance ; let us get rid of the corners 

 and turn the leading edge into a continuous curve (Fig. 458). The 

 leading edge is now doing most of the ^^ork, and the area within 

 and behind is doing little good. Vorticoid air-currents are beating 

 down on either side on this inner area; moreover, air is ''sliding 

 out" below, and tending to curl round the tip and edges of the 



* Note that the aeroplane copies the beetle rather than the bird, as Lilienthal 

 himself points out, in Vom Gleitflug zu Segelflug, Berlin, 1923. 



f On a particular case of the descent of a heavy body in a resisting medium, 

 Camb. and Dublin Math. Jonrn. ix, pp. 145-148, 1854; Sci. Papers, i, pp. 115-118. 

 This elegant and celebrated little paper was written by Clerk Maxwell while an 

 undergraduate at Trinity College, Cambridge. 



