372 



~DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



the mutual action and reaction of the body and wings during the up stroke is 

 that furnished by a partly opened umbrella, whose handle has been intention- 

 ally weighted. If the umbrella thus prepared be dropped from a height, the 



without any chance of return to the surface. In aerial flight, weight is the power which nature has 

 placed at the disposal of the bird for regulating its altitude and horizontal flight, a cessation of the play 

 of its wings, aided by the inertia of its trunk, enabling the bird to approach the earth . In subaquatic 

 flight, levity is a power furnished for a similar but opposite purpose ; this, combined with the partial 

 slowing or stopping of the wings and feet, enabling the diving bird to regain the surface at any 

 moment. Levity and weight are auxiliary forces, but they are necessary forces when the habits of the 

 animals, and the form and mode of applying their travelling surfaces are taken into account. If the 

 aerial hying bird was lighter than the air, its wings would requite to be twisted round to resemble the 

 diving wings of the penguin and auk. If, on the other hand, the diving bird (penguin or auk) was 

 heavier than water, its wings would require to resemble aerial wings, and they would require to strike 

 in an opposite direction to that in which they strike normally. From this it follows that weight is 

 necessary to the bird (as at present constructed) destined to navigate the air, and levity to that destined 

 to navigate the water. If a bird was made very large and very light, it is obvious that the diving 

 force at its disposal would be inadequate to submerge it. If, again, it was made very small and very heavy, 

 it is equally plain that it could not fly. Nature, however, has struck the just balance ; she has made 

 the diving bird, which flies under the water, relatively much heavier than the bird which flies in the 

 air, and has curtailed the travelling surfaces of the former, while she has increased those of the latter. 

 For the same reason, she has furnished the diving bird with a certain degree of buoyancy, and the 

 flying bird with a certain amount of weight — levity tending to bring the one to the surface of the 

 water, weight the other to the surface of the earth, which is the normal position of rest for both. The 

 action of the subaquatic or diving wing of the king penguin is well seen in the annexed woodcut 

 (Fig. 44). 



Fig. a. 



At A, the penguin is in the act of diving, and it will be observed that the anterior or thick margin of 

 the wing is directed downwards and forwards, while the posterior margin is directed upwards and back- 

 wards. This has the effect of directing the under or ventral concave surface of the wing upwards and 

 backwards, the effective stroke being delivered in this direction. The efficacy of the wing in counter- 

 acting levity is thus obvious. At B, the penguin is in the act of regaining the surface of the water, 

 and in this case the wing is maintained in one position, or made to strike downwards and forwards like 

 the aerial wing, the margins and under surface of the pinion being reversed for this purpose. The 

 object now is not to depress but to elevate the body. Those movements are facilitated by the alter- 



