VII 



FLIGHT 187 



diminishing as the bird's position approaches nearer 

 and nearer to the horizontal. This reduction of 

 waste need not, however, mean an absolute gain. 

 He will have to bear in mind that if he reduces the 

 upward inclination of his body to the vanishing point, 

 his course will be inevitably downwards. When the 

 angle becomes extremely acute, nearly all the resist- 

 ance comes in the form of support, but this fact will 

 avail him nothing if the total of resistance be too 

 small. Thus the Rook has to make a calculation : if 

 his velocity be so many feet per second, at what angle 

 must he set himself in order not to lose elevation ? 

 This will vary very much with the pace of the bird. 

 The Swift and the Rook are to one another as an 

 express and a parliamentary train : consequently the 

 Swift can venture upon a much more acute angle than 

 the Rook : consequently he will lose pace less rapidly 

 and will be able to glide further. 1 



Mathematical problems are often simplified as 

 Sir George Cayley has simplified this. The com- 

 plications in Nature are so great that, in many 

 cases, it is necessary to eliminate some of them, 



1 It is estimated that the horizontal is to the vertical resist- 

 ance as the sine of the angle made by the bird's body with the 

 horizon is to the cosine. 



Thus if BA represents the bird's body the relative lengths of 

 AC and CB represent the ratio of the horizontal resistance of 

 the air to the vertical resistance. 



Fig. 51 



