FLIGHT OF BIRDS. 



upper air, with a very slight rise in direction, 

 will easily remove our difficulty. 



If A, B, C, D (Fig. 18) represents a block of 

 the upper air, in which the bird H is quietly 

 gliding, E may represent the position to which 

 it would have moved through perfectly still 

 air after a definite interval, say one second, 

 the direction and distance being represented 

 by the arrow F. Suppose that there is a breeze 

 moving in exactly the opposite direction at 

 the same speed, as indicated by the arrow G. 

 It is obvious that this breeze will carry the 

 bird backwards and upwards just as far as its 

 own gliding had taken it downwards and for- 

 wards, and so its actual position would be 

 unchanged. 



Another puzzle in bird movement which is 

 not often explained is how birds get such rapid 

 forward movement without beating their wings 

 backwards against the air. When an oarsman 

 propels his boat forwards he beats his oars 

 backwards against the water, and the reaction 

 drives the boat on. A paddle steamer does 

 the same with its paddles, and a duck or swan 

 with its feet. But if we carefully watch any 

 bird such as a rook, peewit, or seagull, whose 

 wings move slowly enough for the nature of 

 their movements to be observed, we cannot 

 detect the slightest backward and forward 

 movement of the wings ; it appears to be 

 purely up and down. Such a movement of 



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