264 FLIGHT 
of descending obliquely from the high rock on which the town of 
Constantine is built by a gliding flight; that they usually followed 
a straight line, and, with a constant velocity, progressed thus more 
than a kilometre before reaching the ground. The rate of motion 
was found to be about 20 metres per second, and the trajectory 
was at an angle of 10 degrees with the horizon, which gives a 
descent of about 1 in 5. This corresponds with other observations. 
We now come to the question how a bird guides its motion during 
the act of gliding, and here we must refer to the Law of Avanzini,! 
illustrated by the annexed diagrams, shewing that a plate (AB) 
N i yf DEA 
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Fig. 1. Mie. 2 
falling vertically through the air (as in Fig. 1) encounters the 
maximum of resistance (indicated by the greater length of the 
lower series of arrow-heads) at the centre, the resistance decreasing 
toward the margin, whereas if the direction in which the plate falls 
be oblique (as in Fig. 2), the maximum of resistance is no longer 
at the centre but the fore end of the plate, which therefore has a 
tendency to tilt up. In the case of a bird which has no horizontal 
motion, but is falling slowly with extended wings, it is known that 
the point of maximum upward pressure by the air on the lower 
surface of each wing will correspond with the centre of its area ; 
and the same is the case when a bird gives a downward stroke of 
the wing, if the bird has no forward motion through the air. If, 
however, the bird be gliding forward, the point of maximum 
upward pressure is changed and is placed nearer the anterior 
margin of the extended wing, and the faster the bird is moving the 
further forward is the position of maximum upward thrust. The 
result of this is that any increase of velocity tends to thrust up the 
front part of a gliding bird’s wings; and in the same way and 
1 “Resistenza dei fluidi,” Mem. Istit. Naz. Ital. Bologna, i. p. 199. A more 
recent account of the same subject is that by Lord Rayleigh ‘‘On the Resistance 
of Fluids,” Philos. Mag. ser. 5, ii. p. 430 (1876), ” 
