PEOGRESSION IN OE THEOUGH THE AIE. 



The atmosphere, because of its great tenuity, mobility, and 

 comparative imponderability, presents little resistance to 

 bodies passing through it at low velocities. If, however, the 

 speed be greatly accelerated, the passage of even an ordinary 

 cane is sensibly impeded. 



This comes of the action and reaction of matter, the resist- 

 ance experienced varying according to the density of the 

 atmosphere and the shape, extent, and velocity of the body 

 acting upon it. While, therefore, scarcely any impediment 

 is offered to the progress of an animal in motion, it is often 

 exceedingly difficult to compress the air with sufficient rapidity 

 and energy to convert it into a suitable fulcrum for securing 

 the onward impetus. This arises from the fact that bodies 

 moving in the air experience the minimum of resisfMce and 

 occasion the maximum of displacement Another and very 

 obvious difficulty is traceable to the great disparity in the 

 weight of air as compared with any known solid, this in the 

 case of water being nearly as 1000 to 1. According to the 

 density of the medium so is its buoying or sustaining power. 



2'he Wing a Lever of the Third Order, — To meet the pecu- 

 liarities stated above, the insect, bat, and bird are furnished 

 with extensive surfaces in the shape of pinions or wings, 

 which they can apply with singular velocity and power, as 

 levers of the third order (fig. 3, p. 20),^ at various angles, or 

 by alternate slow and sudden movements, to obtain the 



1 In this form of lever the power is applied between the fulcrum and the 

 weight to be raised. The mass to be elevated is the body of the insect, bat, 

 or bird, — the force which resides in the living pinion (aided by the inertia of 

 the trunk) representing the power, and the air the fulcrum. 



