NATATORES. 425 



resistance of the water, or the string of the kite. This mo- 

 mentum is entirely owing to impulses previously given to the 

 air by means of his wings, and when, owing to the resistance 

 of the air, it has decreased so much that he is no longer able 

 to move with sufficient rapidity to prevent his falling, fresh 

 impulses have to be given. Tor this reason, Albatroses sail 

 much longer in fine than in stormy weather, rain especially 

 soon destroying their momentum, and frequently obliging 

 them to use their wings for propulsion. 



" It is by combining, according to the laws of mechanics, 

 this pressure of the air against his wings with the force of 

 gravity, and by using his head and tail as bow and stern 

 rudders, that the Albatros is enabled to sail in any direction 

 he pleases, so long as his momentum lasts. If, when sailing 

 against the wind, the inclination of his body is such that the 

 upward pressure of the wind against his wings and body just 

 balances the force of gravity, his momentum alone acts, and 

 he sails straight in the 'wind's eye.' If he wishes to ascend, 

 he inclines his body more to the horizon by means of his head 

 and tail. If he wishes to turn to the right, he bends his head 

 and tail slightly upwards, at the same time raising his left 

 side and wing, and lowering the right in proportion to the 

 sharpness of the curve he wishes to make, the wings being 

 kept quite rigid the whole time. To such an extent does he 

 do this that, in sweeping round, his wings are often pointed 

 in a direction nearly perpendicular to the sea; and this 

 position of the wings, more or less inclined to the horizon, is 

 seen always, and only when the bird is turning. It will be 

 observed that, on this principle, an Albatros sailing down 

 wind must necessarily be descending, unless his pace is much 

 greater than that of the air, and such I have found to be 

 invariably the case. 



" It may be objected that the resistance of the air must soon 

 destroy his momentum ; but the fact is that it does not do 

 so. A good illustration of this is seen in an experiment, 



