DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 215 



fig. 18), the floating or buoying area of which greatly exceeds that of some of the llyini* 



beetles (Plate XIII. fig. 16). 



In those animals which fly, as Bats (Plate XIII. fig. 15), Insects (I Mate X 1 1 1, fig. 19), 

 and Birds (Plate XIV.), the travelling-surfaces, because of the extreme tenuity of the air, 

 are prodigiously augmented (in some instances exceeding the actual area of the body to an 

 almost incomprehensible extent (Plate X 1 1 1 . fig. 27). AVhile, therefore, the movements in- 

 volved in walking, swimming, and flying are to be traced in t lie first instance to t lie contrac- 

 tion and relaxation of the muscular, fibrous, or other contractile tissues, and to the bones, 

 when present, and the peculiar forms of their articular surfaces, tin y «'>W to be referred in 

 the second instance to the extent and configuration of the travelling-areas — these on all 



occasions being accurately adapted to the capacity and strength of the animal and the 

 density of the medium on or in which it is intended to progress. Thus the land supplies 

 the resistance, and affords the support necessary to prevent the small feet of land -animals 

 from sinking to dangerous depths, while the water, immensely less resisting, furnishes the 

 peculiar medium requisite for buoying the fish, and for exposing, without danger and to 

 most advantage, the vast extent of surface contained in its ponderous lashing tail,— the air, 

 unseen and unfelt, furnishing that quickly yielding and subtle clement on which the rush- 

 ing pinions of the insect, bat, and bird vibrate with lightning rapidity, with never a grate, 



a jerk, or a jar. 



PROGRESSION IX OR THROUGH THE AIR. 



of 



The atmosphere, because of its great tenuity, mobility, and comparative imponder- 

 ability, presents little resistance to bodies passing through it at low velocities. If, how- 

 ever, the speed be greatly accelerated, the action of even an ordinary cane is sufficient 



to elicit a recoil. 



This comes of the action and reaction of matter, the resistance experienced varying 



according to the density of the atmosphere and the shape, extent, and velocity of the bod) 

 acting upon it. While, therefore, scarcely any impediment is offered to the progress 

 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 this medium experience the minimum of 

 resistance and occasion the maximum of displacement. Another and very obvious diffi- 

 culty 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), and the consequent want of 

 buoying- or sustaining-power which that disparity involves. To meet the.e peculiarities 

 the insect, bat, and bird are furnished with extensive surfaces in the shape of pinions or 

 winos which they can apply with singular velocity and power, as levers of the third order*, 

 at various an-les or by alternate slow and sudden movements, to obtain the necessary 

 degree of resistance and non-resistance. Although the third order of lever is particularly 



* In this form of lever, as every one knows, the power is applied between the fnlcnam and the weight to be raised. 

 The mass to be elevated in the present instance is, I need scarcely remark, the body of the insect, bat, or bmh-the 

 force which resides in the living pinion (aided by the inertia of the trnnk) representing the power, and the thm 



medium of the air the fulcrum. 



2g2 



