GLIDING 21 



(taking his weight as four pounds and four-fifths) 

 his wings would together measure twenty square 

 feet ! This would be a monstrous acreage of wing 

 to raise and lower. But when we have pronounced 

 it monstrous, we have still to answer the question 

 why it is that the big flyer requires, in proportion 

 to his weight, a comparatively very small support- 

 ing surface ? 



Let us imagine the Swallow supplied with wing- 

 area at no more liberal rate per pound weight than 

 that at which the Stork is supplied. Then, taking 

 the Swallow's weight to be about five-sevenths of 

 an ounce, he would have five square inches of 

 wing-surface — two and a half on either side — a 

 miserably poor allowance. A wing so small would 

 be largely made up of margin, and the air would 

 escape at the edges. The gnat has over four and 

 a half square yards of wing for one pound weight. 

 His actual allowance for his almost imponderable 

 insignificance is considerable, but if we make pro- 

 vision for him at the rate at which the Stork is 

 supplied, his wing-surface becomes a mere point, 

 "without parts and without magnitude," to quote 

 Euclid's familiar definition. The air would offer 

 no resistance to so near an approximation to the 

 theoretical point. Here, no doubt, we are getting 

 at the main fact that explains the comparatively 

 small size (when weight is allowed for) of the wings 

 of the great flyers. Since the wing-surface is much 

 larger absolutely the air does not escape so easily 

 at the margins ; each square inch is more effective, 

 since there is less waste. Later on I shall make 

 a further comparison of big birds and small, but 



