254 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



exist; and it is more than probable that when joints are present they are added to 

 augment the power of the wing during its active state, i. e. during flight, quite as much as 

 to assist in arranging the pinion on the hack or side when the wing is passive and the 

 animal is reposing, The flexion of the wing is most obvious when the bird is exerting 

 itself, mid may be detected in birds which skim or glide when they are rising, or when 

 they are vigorously flapping their wings to secure the impetus necessary to the gliding 

 movement. It is less marked at the elbow-joint than at the wrist ; and it may be 

 stated generally that, as flexion decreases, the twisting flail-like movement of the 

 wing at the shoulder increases, and vice versa,— the great difference between sailing 

 birds (see Diagram 18 n, p. 253, and Diagram 19, p. 259) and those which do not 

 sail (Diagram 17, p. 252) amounting to this, that in the sailing birds the wing is 

 worked from the shoulder by being alternately rolled on and off the wind, as in insects ; 

 whereas, in birds which do not glide, the spiral movement travels along the arm as in 

 bats, and manifests itself during flexion and extension in the bending of the joints and 

 in the rotation of the bones of the wing on their axes. The spiral conformation of the 

 pinions, to which allusion has been so frequently made, is best seen in the heavy -bodied 

 birds, as the Turkey, Capercailzie, Pheasant, and Partridge (Plate XV. fig. 70) ; and 



here also the concavo-convex form of the wing is most perceptible (Plate XIV. figs. 31 

 and 32). In the light-bodied, ample- winged birds, the amount of twisting is dimi- 

 nished (Diagram 18 A. a' b', cV ef, p. 253), and, as a result, the wing is more or less 

 flattened (Plate XIV. figs. 29 and 30). 



Power of the Wing — to what owing. — The shape and power of the pinion depend upon one 

 of three circumstances — to wit, the length of the humerus*, the length of the cubitus or 

 forearm, and the length of the primary feathers. In the Swallow the humerus, and in 

 the Humming-bird the cubitus is very short, the primaries being very long ; whereas in 

 the Albatros the humerus or arm-bone is long and the primaries short. When one of 

 these conditions is fulfilled, the pinion is usually greatly elongated and scythe-like (Plate 



XIV. fig. 35, Plate XV. fig. 64)— an arrangement which enables the bird to keep on the 

 win- for immense periods with comparatively little exertion, and to wheel, turn, and 

 Slide about with exceeding ease and grace. When the wing is truncated and rounded, 

 a form of pinion usually associated with a heavy body, as in the Grouse, Quail, Diver, 

 and Grebe, the muscular exertion required and the rapidity with which the wing moves 

 a, re very great, those birds, from a want of facility in turning, flying either in a straight 

 line or making large curves. They, moreover, rise with difficulty, and alight clumsily 

 and somewhat suddenly. Their flight, however, is perfect while it lasts. 



The Goose (Plate XIV. fig. 42) and Duck (Plate XIV. fig. 37), Pigeon (Plate XIV. fig. 33) 

 and Crow (Plate XIV. figs. 41 and 43), are intermediate both as regards the form of the 

 wing and the rapidity with which it is moved. 



The Heron (Plate XIV. fig. 38) and Humming-bird, furnish extreme examples in 

 another direction,-thc Heron having a large wing with a leisurely movement, the Hum- 

 uung-hn-d a comparatively large wing with a greatly accelerated one 



ual'Jtm ^/T^ " le ,° 8th ' bd " S «* Sh0rt » the S ™»°™. <>f derate length in the GalU- 

 GoZ E tit tall ^ \'°7 "7, V" "' e Ga " netS - a " d »»-» ^ "W ». 'he Albatros. In the 



vjuiuui xjagie it 1^ also seen to be of oreat leno-tVi n ir •#/• » ^ . 



great length. —MaegtUwoy't British Bird,, vol. i. p. 30. 



