192 SPECIAL PHYSIOLOGY. 



with these necessities, the wings of the bird operate on the air chiefly in a ver- 

 tical direction ; but, be it observed, owing to the more yielding nature of the 

 hinder edge of the wing, the air escapes under that edge, which is lifted up, so 

 that the efficient action of the stroke is not directly downwards, but downwards 

 and a little backwards in the air. The reaction of that medium takes place 

 against the wings in the opposite direction, that is, chiefly upwards, but 

 slightly forwards ; and the combined result is to sustain, or lift, the centre of 

 gravity of the body of the bird above the tips of the descending wings, and to 

 urge it also forwards. The former act demands a very large expenditure, but 

 the latter a comparatively small outlay of wing force ; The wings, having made 

 their downward stroke, are lifted, and then again de k scend, and so on. The rea- 

 sons why the descending stroke of the wing is more effective than the upward 

 movement, are these : first, the holding power of the wing is increased by the 

 concavity of its under surface, and by the concavity of every quill-feather, also 

 by the overlapping of these, and the locking together of their barbs and bar- 

 bules ; whilst in the upward movement, the air passes off the convex surface 

 of the wing and its chief component feathers, and, as it were, filters through, 

 behind the weaker posterior barbs of each feather, and through their unlocked 

 barbules. Secondly, it is probable, that the area of the wing, owing to its more 

 perfect extension, is slightly greater in its downward movement than during 

 its upward movement ; a condition also favored by the further yielding and 

 bending of the quills and wings in the upward movement, as compared with 

 their stiffness and diminished curvature in the downward stroke. Lastly, 

 force and time being joint elements in the development of a given momentum, 

 it is probable that the downward stroke is accomplished with greater energy 

 and velocity than the upward movement. Be this as it may, without an ex- 

 traordinary difference between the efficient action of the downward and the up- 

 ward movement of the wing, no sustaining, much less a lifting, power would 

 be gained, and the force of gravity would cause the bird to descend to the 

 ground. By rapid strokes of the win^s, slightly rotated and firmly held, in a 

 directly downward direction, the bird is lifted upwards perpendicularly in the 

 air. In hovering over one spot, the wings appear to act directly downwards, 

 but probably they are so rotated forwards, as to counteract the effect of the 

 sliding of the air from behind their posterior edges ; and thus they merely sup- 

 port the bird at one spot in the air. Progressive movement requires in addi- 

 tion, a slightly oblique force exercised, as just described, downwards and a 

 little backwards, so as to produce a reactionary force of the air upwards and 

 a little forwards. The effect of gravity alone, when once the bird is raised 

 sufficiently high in the air, will produce, owing to the easier escape of the air 

 behind the weak edge of the wings, a forward but gradually descending move- 

 ment, which is known as sailing through the air. In gliding or skimming 

 obliquely downwards through the air, some birds use their wings outstretched, 

 after the manner of a parachute, whilst others alternate the flying and the 

 parachute movement ; some fly continuously, others by jerks, rising by rapid 

 movements of the wings, and falling when these are quiet. The tail operates, 

 in regulating or checking the descent of the bird, obliquely, by gravity. It is 

 also employed as a rudder by which to steer the bird, or to cause it, during 

 active flight, to ascend or descend in the air ; when the tail is bent downward, 

 the resistance of the air beneath and in front of it, causes the head of the bird 

 to ascend ; when it is raised, on the same principle, the bird is made to descend. 

 The direction of the flight, to one side or the other, is said by some to be caused 

 by the more rapid vibration, and perhaps by the changed position of the oppo- 

 site wing, rather than, as supposed by others, by any lateral action of the tail. 

 Some birds, especially sea-birds, have a remarkable power of flying, or gliding, 

 on their sides in the air, or of turning completely over ; tumbler-pigeons make 

 summersaults over and over again. The protrusion, or retraction, the eleva- 

 tion, or depression, or the lateral movement of the head and neck, which will 

 shift the centre of gravity in corresponding directions, must also aid in deter- 

 mining the direction of flight. 



The flight of some birds is very rapid, reaching, it is said, to ninety miles 

 an hour, in the Eider duck, and even to one hundred and one hundred and 

 fifty miles an hour, in the case of certain hawks and falcons. The wings, the 



