CONTRIVANCE A NECESSITY. 139 



must be light enough in substance not to add an 

 excess of weight to the already heavy body of the 

 bird ; next, it must be strong enough in frame to 

 withstand the pressure which its own action on the 

 air creates. The first of these conditions is met by 

 an exact adjustment of the size or area of the wing 

 to the size and weight of the bird which it is to lift. 

 The second and the third conditions are both met 

 by the provision of a peculiar substance, feathers, 

 which are very light, and very strong ; whilst the 

 only heavy parts of the framework, namely, the 

 bones in which the feathers are inserted, are limited 

 to a very small part of the area required. 



But there is another difficulty to be overcome — 

 a difficulty opposed by natural laws, and which 

 can only be met by another adjustment, if possible 

 more ingenious and beautiful than the rest. It is 

 obvious that if a bird is to support itself by the 

 downward blow of its wings upon the air, it must 

 at the end of each downward stroke lift the wing 

 upwards again, so as to be ready for the next. 

 But each upward stroke is in danger of neutral- 

 ising the effect of the downward stroke. It must 

 be made with equal velocity, and if it required 



