142 THE REIGN OF LAW. 



against the elastic air, might enable animals pos- 

 sessed of such power to lift themselves from the 

 ground and nothing more. There is a common toy 

 which lifts itself in this manner from the force 

 exerted by the air in resisting, and reacting upon 

 little vanes which are set spinning by the hand. 

 But the toy mounts straight up, and is incapable of 

 horizontal motion. So, there are many structures 

 of wing which might enable animals to mount 

 into the air, but which would not enable them to 

 advance or to direct their flight. How, then, is 

 this essential purpose gained ? Again we find an 

 appeal made to natural laws, and advantage taken 

 of their certainty and unchangeableness. 



The power of forward motion is given to birds, 

 first by the direction in which the whole wing 

 feathers are set, and next by the structure given to 

 each feather in itself. The wing feathers are all set 

 backwards, that is, in the direction opposite to that 

 in which the bird moves, whilst each feather is at 

 the same time so constructed as to be strong and 

 rigid toward its base, and extremely flexible and 

 elastic towards its end. On the other hand, the 

 front of the wing, along the greater part of its 



