200 ANIMAL MKCIIAMSM. 



which impels tin- insect forwards. This resultant acts in the 

 descent of the wing 1 , as well as iu its upward movement, so 

 that each part of the oscill itiou of tin- wing lias ;n 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 oppose, 1 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 l>y 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, who.se movement is continuous, 

 and alwnvs in the same direction. 



FJO. 85 R '1'i'i.xuiituUciii cf thu chiiugus in the i>l.lne of the insert's wing. 



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

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

 fig. N.>, in which the arrows indicate the direction of the 

 course, of (he 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- 

 ti^-rew in his work on flight, to prove how far the ideas of 

 tliis English writer differ from ours 



The trajectory of the wing is represented by l)r. 1'etti- 

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

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



