

DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 2G5 



miraculous journeys at present performed by the denizens of the air rendered impos- 



sible 



V. In the bat, the only mammal at present known to fly, the surfaces employed in 

 locomotion are enormously increased, from the fact of their being destined to operate on 

 the air, at once the lightest and rarest of the media adapted for transit. The bat is, 

 therefore, assimilated in structure to the bird, as the sea-mammals are assimilated to the 

 fish. The bat secures the degree of resistance and non-resistance necessary for propul- 

 sion by contracting and raising the wing in its plane of least resistance on the one hand, 

 and by expanding and depressing it in its plane of greatest resistance on the other. The 

 bones of the arm and hand supply the degree of rotation necessary for inci asing and dimi- 

 nishing the extent of surface— this being greatest when the wing makes an angle of some- 



n like 30 



with the horizon, when the effective or down stroke is given. The bat i 

 with°the insect, and differs from the bird, in flying by the aid of a continuous memb 



g and 



..Me it agrees with the bird, and differs from the insect, in alternately 



folding its pinion *. It has, however, this feature in common, and it is significant-it 

 applies its wing to the air as an inclined plane, the plane travelling from the point of 

 least resistance when the wing is flexed, to that of greatest resistance when it is extended, 

 and when it makes an angle of 30°, or thereabouts, with the horizon. The peculiar power 



residing in the inclined plane formed by the wing is in part referable to the weight ot 

 the bat, the inertia of the trunk acting in conjunction with the wing upon the — 



to 



produce a recoil, which carries the whole in an upward and forward direction 



The sam< v 



holds true of the insect and bird 



the helix 



VI. In the insect, the wing is more or less twisted upon itself, as m 

 screw, the anterior margin being greatly thicker than the posterior, which yields s igl.t y 

 to prevent shock. The wing, whatever its position during the period of repose, trareta 

 from the point of least resistance (usually its plane of restt) till it makes an anje 

 bordering upon 30° with the horizon, when the effective or down stroke -J** » 

 that it, too, acts as an inclined plane, the angle being increased during exten^ when 

 the effective stroke is given, and decreased during flexion when it trave s **» » 

 starting-point. The double hinge, or compound joint, by which the Wing is alt., lied to 

 the trunk, secures the rotation or twisting necessary for this purpose. 



VII. In the bird the wing is more high* JfT^^SSSS ' £ 



fact, presents the distinguishing features of the wings of the msect ana 



it reLues that of the bat in having a bony skeleton funnshed ^^J^SSL 



. • J. ~<**A in rvrp^Pntlli * & Compact ltw&uii 



museles, neither of which are found m the insect, and ™ P es ™ ■ ' ho 



surface during extension, when the effective or downs £to» _ n^ M ^ ^ 



from the hat's wins durins flexion, when the back stioke is 3 iven, 



. . , t MJ™. in flexion of the ning, and of unfold.ng 



VI «.****£, 



or opening out during extension. In the Albatros, the wing m some reS P e ^ ^ sU bcoverts, which do not admit 

 being habitually extended, and because of the great number of the ™ ng ~ C °'" ^ _ art3 of the w i ng are used like 

 of the wind passing between the feathers. In Alucita hexadactyla, the l principle involved always 



the separate feathers of the win, of the bird. These afford examples of modmcatmn, p 



remaining the same, 

 t The Draironflie 



