FORM OF THE BIRD. 



219 



effect was produced similar to that of a rudder, which turned 

 up the anterior extremity of the little apparatus, and gave it 

 an ascending course. Immediately, the weight which was 

 the generating force of its gliding movement through the air 

 began to retard it ; in proportion as it rose, it lost its velo- 

 city until it reached the point of rest. After that, a downward 

 movement commenced, then an ascent in the opposite direc- 

 tion, so that the paper descended to the ground by successive 

 oscillations. 



If we give the apparatus a slight concavity downwards, 

 the opposite effect is produced; we see (fig. 92), at a certain 



Fig. 92.— The right hand corner of the pbne of the angle has been bent 

 downwards. After a parabohc descent, the apparatus falls very rapidly 

 in a perpendicular direction. 



moment, the trajectory turns abruptly downwards, and the 

 •falling body strikes the ground with considerable violence. In 

 this second case, when the rudder-like effect is produced, the 

 new direction has in its favour the weight which hastens the 

 fall of the little instrument, as in the former experiment it 

 rendered the re- ascent more slow. 



We have dwelt upon these effects, because they often occur 

 in the flight of birds. They are mentioned in the old treatises 

 on falconry, which describe the evolutions of birds used in 



