200 



ANIMAL MECHANISM. 



which impels the insect forwards. This resultant acts in the 

 descent of the wing, as well as in its upward movement, so 

 that each part of the oscillation of the wing has an action 

 favourable to the propulsion of the animal. 



An effect is produced analogous with that which takes place 

 when an oar is used in the stern of a boat in the action of 

 sculling. Each stroke of the oar, which presents an inclined 

 plane to the resisting water, divides this resistance into two 

 forces : one acts in a direction opposed to the motion of the 

 oar, the other, in a direction perpendicular to that movement, 

 and it is the latter which impels the boat. 



Most of the propellers which act in water overcome the 

 resistance of the fluid by the action of an inclined plane. 

 The tail of the fish produces a propulsion of this kind ; that 

 of the beaver does the same, with this difference, that it 

 oscillates in a vertical plane. Even the screw may be con- 

 sidered as an inclined plane, whose movement is continuous, 

 and always in the same direction. 



Fio. 85.— Representation of the changes in the plane of the insect's wing. 



If we wish to represent the inclination of the plane of the 

 wing at the different parts of its course, we shall obtain 

 fig. 85, in which the arrows indicate the direction of the 

 course of the wing, and the lines, whether dotted or full, 

 show the inclination of its plane. 



After this, we need only show the figure traced by Dr. Pet- 

 tigrew in his work on flight, to prove how far the ideas of 

 this English writer differ from ours. 



The trajectory of the wing is represented by Dr. Petti- 

 grew by means of fig. 86. Four arrows indicate, according 

 to this writer, the direction of movements in the different por- 



