CONTRIVANCE A NECESSITY. 135 



capable of flight must present its maximum of sur- 

 face to the resistance of the air in the perpendicular 

 direction, and its minimum of surface in the hori- 

 zontal direction. Now, both these conditions are 

 satisfied (i) by the great breadth or length of sur- 

 face presented to the air perpendicularly in a bird's 

 expanded wings, and by (2) the narrow lines pre- 

 sented in its shape horizontally, when in the act of 

 forward motion through the air. But something 

 more yet is required for flight. Great as the re- 

 sisting force of air is, it is not strong. enough to 

 balance the Force of Gravity by its mere pressure 

 on an expanded wing — unless that pressure is 

 increased by an appeal to yet other laws — and 

 other properties of its nature. Every sportsman 

 must have seen cases in which a flying bird has 

 been so wounded as to produce a rigid expansion 

 of the wings. This does not prevent the bird from 

 falling, although it breaks the fall, and makes it 

 come more or less gently to the ground. 



Yet further, therefore, to accomplish flight, 

 another law must be appealed to, and that is the 

 immense elasticity of the air, and the reacting 

 force it exerts against compression. To enable an 



