398 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



In other words, the air pressure is removed above but maintained 

 beneath the pinion, so that it is supported theoretically by a force 

 approaching 14.7 pounds per square inch of surface. Of course, this 

 vacuum is by no means complete and is of very brief duration, but 

 it is obvious that the lifting power of the air beneath is ample to 

 support a much larger bird than a gull on the same wing area. 



The displaced air can not rush in so quickly in the wake of a 

 large wing as in the wake of a small one. This explains why a gull 

 is able to support itself in the air with only two strokes per second, 

 while a sparrow, which really has a greater wing expanse in pro- 

 portion to its weight than a gull, must take 13 strokes per second 

 (Marey, 1890, p. 100). A large wing is intrinsically a more efficient 

 instrument of flight than a small wing, without reference to the 

 weight of the bird to be supported. 



During the beat of the wings there is a certain forward and back- 

 ward, as well as up and down, motion, so that the wing tip describes 



C 



Fig. 1. — Theoretical trajectory of the wing tip, on a somewhat shortened horizontal scale 



an ellipse with reference to the body of the bird, or, owing to the 

 forward movement of the bird, a series of loops (fig. 1), which be- 

 come more and more nearly closed with increasing acceleration of 

 flight. It is possible even that the trajectory of the wing tip is a 

 sort of figure 8, as Pettigrew (1847, pp. 15ff.) has insisted, and 

 Marey (1890, p. 140) has described for the crow; but the presence 

 of a secondary loop can not be determined by observation, and seems 

 rather doubtful. 



Two phases of the loop described by the wing are to be seen in 

 Plate 2, C. The upper bird shows the wings advanced on the down 

 stroke (position A, fig. 1), while the lower bird has them retired on 

 the up stroke (position B, fig. 1). Plate 2, B, shows the wings with 

 the front margins almost vertical, as they would appear at position 

 C, Figure 1. 



The effect of advancing the wings farther than normal is to ro- 

 tate the front margins upward, so that the ventral surface is directed 

 anteriorly, thus retarding forward flight. This is well shown in 



