218 



ANIMAL MECHANISM. 



The trajectory passed through by this little instrument will 

 be situated in a vertical plane, if the two halves of the appa- 

 ratus are perfectly symmetrical ; but if they are not, it will 

 turn towards the side in which while it cuts the air it finds 

 the greater resistance. These effects, which are easily un- 

 derstood, are identical with those which the resistance of the 

 rudder causes in the advancing motion of a ship. They can 

 also be produced in a vertical direction ; so that the trajectory 

 of the apparatus may be a curve with its concavity above or 

 below, as the case may be. 



Every thin body which is curved tends to glide upon the 

 air according to the direction of its own curvature. 



Fig. 91.— We have turned back the rit?ht hand corner of the two planes 

 which form the angle. After a descent in a parabolic curve, the apparatus 

 rises again, as shown by the dotted trajectory. 



If we turn back either the anterior or posterior edge of our 

 little apparatus, we shall see it at a given moment of its 

 descent rise in opposition to its own weight, but it will soon 

 lose its upward movement (fig. 91). Let us consider what 

 has taken place. 



So long as the paper descended with but slight rapidity, 

 the effect of its curvature was not perceptible, because the 

 air resists surfaces only in the ratio of the velocity with which 

 they move. But when the rapidity was sufficiently great, an 



