vil 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.t 
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 thé horizontal resistance of 
the air to the vertical resistance. 
