RE-ACTIONS DURING FLiaHT. 



271 



of the wing ; the glass 

 plate moves from right to 

 left; the tracing must be 

 read from left to right. 

 The head o£ the bird is 

 turned towards the left, 

 its 'flight is in the direction 

 pointed out by the arrow. 



We may divide this 

 figure into a series of 

 portions by means of ver- 

 tical lines passing through 

 homologous points, whe- 

 ther we let fall these per- 

 pendiculars from the top 

 of the loops, or from that 

 of the simple curves, as 

 has been done at the points 

 a and e. Each of these 

 portions will enclose toler- 

 ably similar elements, with 

 the exception of their un- 

 equal development in the 

 different points of the 

 figure : let us neglect this 

 detail for the present. 



It is evident that the 

 periodical return of similar 

 forms corresponds with the 

 return of the same phases 

 in a revolution of the bird's 

 wing. The portion a e will, 

 therefore, represent the dif- 

 ferent movements of the 

 bird in one and the same 

 revolution. 



Let us remember that in 

 the curve which we analyse, 

 all the movements are con- 

 trary to those really per- 

 formed by the bird. The 



