LOCOMOTION IN GENERAL. 107 



dragon-flipspxpelfrom their intestines a veiy strong jet of liquid, 

 and acquire, by this means, a rapid and forcible impulse. 



The oar is found in many insects which move oil the sur- 

 face of the water. A contrivance is employed by other 

 animals, which resembles the action of an oar used at the 

 stern of a boat in the process called sculling. To this 

 latter motive power may be referred all those movements 

 in which an inclined plane is displaced in the liquid, 

 and finds in the resistance of the water, which it presses 

 obliquely, two component forces, of which one furnishes a 

 movement of propulsion. This mechanism will require some 

 explanation ; it will be found ia its proper place, with all 

 the developments which it affords. 



Aerial locomotion. This mechanism is still the same ; the 

 motion of an inclined plane, which causes motion through 

 the air. The wing 1 , in fact, in the insect as well as in 

 the bird, strikes the air in an oblique manner, repels it in a 

 certain direction, and gives the body a motion directly oppo- 

 site. With the exception of certain birds which spread their 

 wings to the wind, and which, hr vering thus without any 

 other effort than simply steering, have received the picturesque 

 name of hovering or sailing birds (oiseaux voiliers), all 

 animals move forward only by an (ffort exerted between t\vo 

 masses unequally movable. It can be easily understood that 

 if one of these points where the force is applied is absolutely 

 fixed, the other alone will receive will cut diminution the 

 motive work developed ; such is the condition of terrestrial 

 locomotion on soil perfectly solid. But we can understand 

 also that the softness of the ground constitutes a condition un- 

 favourable to the utilization of the force employed, and that 

 the extreme mobility both of the air and the water offer still 

 less favourable conditions for swimming or flight. 



But this mobility of the point of resistance varies with the 

 rapidity of the movement; so that a certain stroke of the 

 wing or the oar, which would be without effect if produced 

 slowly, would become efficacious by its very rapidity. 



In different kinds of locomotion, the resistance which it 

 is necessary to overcome in order to displace the body, does 

 not vary less than that which serves as an external point of 



