FLIGHT OF BIRDS. 



is thus a continual up and down movement 

 of the body in its progress forward. 



C and D are diagrams of the same positions 

 as seen from the side, the parallel bars in C 

 showing the slope of the upper surface and 

 those in D the slope of the under surface. 



The bars of slope in C show that when the 

 wing beats downwards in the position A or 

 C, the lower surface strikes the air obliquely, 

 not squarely, because the wing is in an oblique, 

 not a square, position. Now we know what 

 happens when the air is struck obliquely by 

 the oblique surface of a fan. The air is driven 

 not merely round with the fan, but forwards 

 from it, the combined movement being not 

 far from perpendicular to the striking surface. 



So the wing, beating downwards with a 

 surface which looks backwards as well as down- 

 wards, drives the air backwards as well as down- 

 wards. By reaction the wing is thrust forwards 

 as well as upwards, carrying the bird in the 

 same direction. Thus we get the forward 

 movement of the bird out of the purely down- 

 ward beat of the wing. 



In the diagrams the slope of the wing 

 backwards, which produces this movement, 

 has been considerably exaggerated for the sake 

 of clearness. In reality it is much less, a very 

 slight obliquity being sufficient to produce 

 the forward movement. In fact, as we saw 

 in the case of the ice-boat's sails, the smaller 



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