PETTIGREW ON THE MECHANISM OF FLIGHT. 257 



and Flexion. — The wing, as has been explained, is recovered or drawn off the wind 

 principally by the contraction of the elastic ligament extending between the joints, 

 so that the pinion during flexion enjoys a certain degree of lvpose. The time occu- 

 pied in recovering is not lost so long as the wing makes an angle with the horizon 

 and the bird is in motion, it being a matter of indifference whether the wing acts on the 

 air, or the air on the wing, so long as the body bearing the latter is under weigh ; and 

 this is perhaps the chief reason why the Albatros, which is a very heavy bird*, can 

 sail about for such incredible periods without apparently moving the wings at all. 

 Captain Hutton thus graphically describes the sailing of this magnificent bird : — " The 

 flight of the Albatros is truly majestic, as with outstretched motionless wings he sails 

 over the surface of the sea — now rising high in air, now with a bold sweep, and wings 

 inclined at an angle with the horizon, descending until the tip of the lower one all but 

 touches the crest of the waves as he skims over them"f. 



The wing of the bird may therefore be said to act in four different ways : 



1st, during extension, when it gradually makes an angleof 30°, or thereabouts, with 

 the horizon. 



2nd, during the downward stroke, when it maintains, or nearly maintains, the angle 

 of 30° with the horizon, and makes a variable angle with the body. 



3rd, during flexion, when the angle made by the wing with the horizon is reduced, 



preparatory to making the return stroke. 



4th, during the return or up stroke, when the pinion acts at a gradually decreasing 

 angle, in virtue of its being carried against the wind by the body of the bird which is in 

 motion. The wing, as will be seen from this account, acts as a sustainer and partial 

 elevator during its ascent, by being passively forced against the wind by the momentum 

 acquired by the mass bearing it. 



Birds of Flight separable into four kinds.— Birds of flight may be conveniently divided 

 into four kinds : — 



1st, such as have heavy bodies and short wings with a rapid movement (Plate XIV. 



figs. 31 and 32). 



2nd, such as have light bodies and large wings with a leisurely movement (Plate 



XIV. fig. 38). 



3rd, such as have heavy bodies and long narrow wings with a decidedly slow move- 

 ment (Diagram 18 d, p. 253 ; Diagram 19, p. 259) ; and 



4th, such as are intermediate with regard to the size of body, the dimensions of wing, 



and the energy with which it is driven (Plate XIV. fig. 30). 



They may be subdivided into those which float, skim, or glide (Plate XIV. fig. 28), 

 and those which fly in a straight line (Plate XIV. figs. 31, 37 and 12) and irregularly 



(Plate XIV. figs. 36 and 39). 



The Pheasant, Partridge (Plate XIV. figs. 31 and 32), Grouse, and Quail furnish good 

 examples of the heavy-bodied, short-winged birds. In these the wing is rounded and 

 deeply concave. It is, moreover, wielded with immense velocity and power. 



The average weight of the Albatros, as given by Gould, is l/lbs.-I^, 2nd series, vol. i. 1*65, p. 295. 



"On some of the Birds inhabiting the Southern Ocean," byCapt.F. W. Hutton, 'Ibis,' 2nd series, vol. i. I865,p.282. 



