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, Avith 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 Avith 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) 



* 



B A 



AAAAA/N/N4\'|^ 



AAAAAAAAA 



B 



Fio. 1. 



Fio. 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 arroAv-heads) at the centre, the resistance decreasing 

 toward the margin, whereas if the direction in Avhich 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 Avhich has no horizontal 

 motion, but is falling slowly with extended wings, it is knoAvn that 

 the point of maximum upward pressure by the air on the lower 

 surface of each Aving Avill correspond Avith the centre of its area ; 

 and the same is the case when a bird gives a downAvard stroke of 

 the wing, if the bird has no forAvard motion through the air. If, 

 hoAvever, the bird be gliding forAvard, 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 forAvard is the position of maximum upAA'ard thrust. The 

 result of this is that any increase of velocity tends to thrust up the 

 front part of a gliding bird's Avings ; and in the same Avay and 



^ "Resistenza dei fluidi," Mem. Mil. Nnz. If.al. 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). 



