iSS THE STRUCTURE AND LIFE OF BIRDS chap. 



if the problem is to be tackled at all. Every one is 

 familiar with the pipes of different dimensions in 

 arithmetic books which are opened at various times, 

 some of them filling, some of them emptying a cistern, 

 the problem being to discover at what precise time 

 the cistern will be full or empty. Such problems give 

 healthy exercise to the brain, but we must not suppose 

 that the behaviour of water passing through pipes is 

 a thing that can be absolutely predicted. The present 

 calculation shows us general principles. But, since it 

 does not take into account the resistance of the air to 

 such an irregular surface as that of a bird's wing, it 

 does not enable us, in the case of a particular bird, to 

 fix the exact angle at which he must set himself, if he 

 wishes, having attained a certain velocity, to glide 

 onward and maintain his level. 



Though the resistance offered by the air to surfaces 

 like those of a wing cannot be accurately measured, 

 yet it is possible to obtain some notion of its amount. 

 If you hold an umbrella so that the inside faces a 

 strong breeze, it feels a great strain, and is likely to 

 give at every point. If, on the other hand, the out- 

 side meets the blast, the air passes harmlessly off its 

 slippery convexity. Herr Lilienthal, the German 

 engineer, who has sailed through the air a distance of 

 over 500 yards with only a slight descent, once, as he 

 was carrying his wings to the place of trial, was 

 cheered by the fact that the air gathered in their 

 curved under-surfaces and relieved him of all the weight. 

 If we take a single big wing feather and wave it 

 through the air, we feel that the resistance varies 

 according as we turn the concave or convex surface to 



