374 DK PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 



stroke, as seen at 4 of figure 19, Plate XIV., and at figure 15, Plate XIII. ; 

 and it is no doubt this circumstance which has induced hasty generalisers to 

 deny that the wing is flexed during the up stroke. This is a pardonable mis- 

 take, as the wing in such cases may be actually extended for two-thirds of the 

 up stroke. When the wing is fully flexed and elevated as a short lever, its 

 rowing feathers are separated and opened up, and the bird draws largely upon 

 its vital resources. When, on the other hand, the wing is elevated as a long 

 lever, and is wielded in one piece, after the manner of the insect wing, the bird 

 takes advantage, to a great extent, of the numerous mechanical adaptations 

 with which nature has endowed it. The flight of the albatros furnishes the 

 best example. The opening up of the feathers during the up stroke facilitates 

 the ascent of the pinion, and permits a more rapid action. The separation of 

 the feathers is, however, not necessary to successful flight, the bat flying 

 remarkably well by the aid of a continuous membrane which, as is well known, 

 is destitute of feathers. 



The Wing Vibrates Unequally on either Side of a given Line. — The wing, during 

 its vibration, descends further below the body than it rises above it. This is 

 necessary for elevating 'purposes. In like manner the posterior margin of the wing 

 (whatever the position of the organ) descends further below a given line than 

 it ascends above it. This is requisite for elevating and propelling purposes, the 

 under surface of the wing being always presented at a certain upward angle to 

 the horizon, and acting as a true kite. This view is fully explained at -p. 345. 

 If the wing oscillated equally above and beneath the body, and if the posterior 

 margin of the wing vibrated equally above and below the line formed by the 

 anterior margin, much of its elevating and propelling power would be sacrificed. 

 The tail of the fish oscillates on either side of a given line, but it is otherwise 

 with the wing of a flying animal. The fish is of nearly the same specific 

 gravity as the water, so that the tail, as a rule, only propels. The flying 

 animal, on the other hand, is very much heavier than the air, so that the wing 

 requires both to propel and elevate. The wing to be effective as an elevating 

 organ must consequently be vibrated rather below than above the centre of 

 gravity ; at all events, the intensity of the vibration should occur rather below 

 that point. In making this statement, it is necessary to bear in mind that the 

 centre of gravity is ever varying, the body rising and falling in a series of curves 

 as the wings ascend and descend. 



To elevate and propel, the posterior margin of the wing must rotate round 

 the anterior one, the posterior margin being, as a rule, always on a lower level 

 than the anterior one (vide pages 414, 415, and 416). By the oblique and more 

 vigorous play of the wings under rather than above the body, each wing expends 

 its entire energy in pushing the body upwards and forwards. Fig. 12, page 342, 

 will illustrate my meaning. Let the oar x, s, represent the wing. If the wing 



