HOW INSECTS FLY. XI 



under side. Hence, there are two sorts of effects; in the 

 insect the up and down strokes are active; in the bird, the 

 lowering of the wing is the only active period, though the 

 return stroke seems to sustain the bird, the air acting on the 

 wing. The bird's body is horizontal when the wing gives a 

 downward stroke; but when the beat is upward, the bird is 

 placed in an inclined plane like a winged projectile, and mounts 

 up on the air by means of the inclined surfaces that it passively 

 offers to the resistance of this fluid. 



In an insect, an energetic movement is equally necessary to 

 strike the air at both beats up and down. In the bird, on the 

 contrary, one active beat only is necessary, the down beat. It 

 creates at that time all the motive force that will be dispensed 

 during the entire revolution of the wing. This difference is due 

 to the difference in 

 form of the wing. 

 The difference be- 

 tween the two 

 forms of flight is 11. Trajectory of an insect's wing, 



shown by an inspection of the two accompanying figures (11, 

 12). n insect's wing is small at the base and broad at the end. 

 This breadth would be useless near the body, because at this 

 point the wing does not move swiftly enough to strike the 

 air effectively. The type of the insectean wing is designed, 



then, simply to 

 strike the air. 

 But in the bird the 

 wing plays also a 

 passive role, i. e., 

 12. Trajectory of a bird's wing. it receives the pres- 



sure of the air on its under side when the bird is projected 

 rapidly onward by its acquired swiftness. In these conditions 

 the whole animal is carried onward in space ; all the points of 

 its wing have the same velocity. The neighboring regions 



