426 BIRDS OP AUSTRALIA. 



common in lecture-rooms a few years ago, by which the 

 rotation of the earth was demonstrated by means of a pen- 

 dulum, composed of a metal ball suspended by a long string 

 from the ceiling of the lecture-hall. The impetus obtained by 

 causing the metal ball to fall through the space of a few feet 

 only was sufficient to keep the pendulum swinging, with a 

 velocity but little diminished, for the greater part of an hour, 

 notwithstanding the resistance of the sand, which the point of 

 the pendulum had to cut through twice during each vibration. 

 The resistance of the air is well known to depend on the shape 

 and velocity of the moving body, and to increase in proportion 

 much more rapidly than the velocity increases. Tor this 

 reason a properly-shaped body and a low velocity are required 

 to reduce it to a minimum. A certain amount of weight is 

 also necessary to give a bird momentum sufficient to overcome 

 the resistance for a certain time, and wings are required of 

 sufficient expanse to support it as it sails slowly through the 

 air. These conditions are admirably carried out in the 

 Albatros ; its expanse of wing is perhaps greater than that of 

 any other bird, and its weight, 15 lbs. and upwards, is very 

 large. Its shape, also, Avhen the neck is stretched out, as in 

 flying, approaches very nearly to that of Newton's solid of 

 least resistance, while more than one voyager has remarked 

 the slowness with which it sails past. The Petrels I have 

 mentioned sail very nearly in proportion to their size and 

 weight. The Stormy Petrel never sails ; the Cape Pigeon 

 only for a very short time, perhaps a minute ; the ' Night- 

 Hawk ' much longer, often between five and ten minutes ; 

 while the Albatros, as I have before mentioned, sails sometimes 

 for an hour, 'rising and falling,' says Dr. Bennett, 'as if 

 some concealed power guided its various motions, without 

 any muscular exertion of its own,' but which we must only 

 look upon as another illustration of the small resistance offered 

 by the air to the passage of a properly-shaped heavy body 

 moving through it with a low velocity." / 



