ANIMAL FLIGHT III 



the warm regions the appearance of clouds which obscure the 

 sun promptly weakens the force of the ascending columns of 

 air and soon cause all the soaring birds to flap and to return 

 to the ground. With us in the north only a few birds, mostly 

 eagles, hawks and vultures, soar, and these only on warm, 

 bright and sunny days. 



Many birds, such as partridges, pheasants, quail, tinamous, 

 etc., w^hen startled fly diagonally upward with great violence 

 to a considerable height and then glide downward to a place 

 of safety, and most of the larger birds glide more or less when 

 approaching the ground or a perch. This gliding has been 

 developed not only in the direction of soaring as just described, 

 but also into a combination of ghding and soaring — mostly 

 gUding — which is characteristic of the flight of a very large 

 number of sea birds. Many of these are such adepts that they 

 can glide all day and never flap their wings. The albatross is 

 the most marvellous of all the gliders; he courses back and 

 forth over the waves, always keeping close to the water, for 

 hour after hour with his long narrow wings extended almost 

 motionless. 



Waves are rows of little hills stretching across the wind. 

 The wind on striking one of these rows of hills is deflected 

 upward with considerable force, and it is by taking advantage 

 of these strong updraughts that the albatross is able to glide 

 perpetually. When flying with the wind the albatross rapidly 

 loses altitude, so he must frequently turn back into the wind 

 again to allow the updraughts from a few waves to raise him 

 anew to the required height. His course to leeward, or down 

 the wind, is therefore a series of loops with long gliding inter- 

 vals between, and his course across the wind is a similar series 

 of loops. As a steamer plows its way along, the air behind it 

 is drawn under the stern with such force as to rise into a 

 column of considerable height just behind it. On this column 

 the albatrosses frequently balance themselves, appearing per- 

 fectly motionless except for the movement of their heads, 

 traveling at the same rate as the ship, being kept up and 



