PROGRESSION IN OR THROUGH THE AIR. 



197 



would be inevitably opened, and the integrity of the wing 

 impaired by a downward and backward movement. 



6 th, The disposition of the articular surfaces of the wing 

 (particularly that of the shoulder-joint) is such as to facilitate 

 the downward and forward movement, while it in a great 

 measure prevents the downw^ard and backward one. 



6 th and lastly. If the wing did in reality strike downwards 

 and backwards, a result the converse of that desired would 

 most assuredly be produced, as an oblique surface which 

 smites the air in a downward and backward direction (if 

 left to itself) tends to depress the body bearing it. This is 

 proved by the action upon the air of free inclined planes, 

 arranged in the form of a screw. 



The Wing acts as an Elevator, Pro]peller, and Sustainer, both 

 during extension and flexion, — The wing, as has been ex- 

 plained, is recovered or drawn off the wind principally by the 

 contraction of the elastic ligaments extending between the 

 joints, so that the pinion during flexion enjoys a certain 

 degree of repose. The time occupied 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 the chief reason why the albatross, which is a very heavy 

 bird,^ can sail about for such incredible periods without flap- 

 ping the wings at all. Captain Hutton thus graphically 

 describes the sailing of this magnificent bird : — The flight of 

 the albatross 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."^ 



Birds of Flight divisible into four kinds : — 



1st, Such as have heavy bodies and short wings with a 

 rapid movement (fig. 59, p. 126). 



1 The average weight of the albatross, as given by Gould, is 17 lbs. — Ibis, 

 2d series, vol. i. 1865, p. 295. 



2 On some of the Birds inhabiting the Southern Ocean," by Capt. F. W. 

 Hutton.— Ibis, 2d series, vol, i. 1865, p. 282. 



