296 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



to a partridge for control when he is using a high angle of incidence 

 in gliding flight; but they are so very long that one can not help 

 suspecting that their unusual shape is in some way connected with 

 the characteristic fast-flapping flight of all game birds. In the wing 

 of a partridge all six slots extend inwards for over one-third of the 

 span of the wing, which may, therefore, be considered as consisting 

 of two sections, the slotted and the solid. The question is, " How 

 does the slotted section behave under the conditions of the extremely 

 rapid beat of these birds? " 



Before attempting to answer that question, it is necessary to run 

 quickly through the action of a wing in simple, straightforward, 

 flapping flight. The most important thing to remember is that the 



U|»p<r cover r j-e<3Lm<rs 



Under coveTTs 



FiGUUE 32. — Under surface of a partridge's right wing-tip. Tlie un- 

 emarginated parts of the feathers are shaded where they overlap 



force produced by the reaction of displaced air must act, for the 

 most part, upwards to counteract gravity; but also in a slightly 

 forward direction to overcome the comparatively weak force of 

 the resistance of the air to the passage of the bird's body. 



For the sake of argument, let us imagine a case in which the 

 required direction of total reaction is 10° forward of the vertical. 

 To obtain it, the blades of the wings must lie in a i^lane tilted 10° 

 (approximately) forward of the horizontal. The inclination of that 

 plane governs the direction in which the wings must move through 

 the air, for the air stream created by their movement must strike 

 them at a suitable angle of incidence. Suppose that this angle is 

 10° ; then the wings must move forward through the air on a path 

 inclined at 20° below the horizontal, as shown in Figure 33. This 

 gradient path is a combination of the forward movement of the 

 bird through the air and the downward movement of the wings 

 themselves. 



