FLIGHT 263 



faster than this, about 35 feet per second according to Cayley, and 

 can glide horizontally for a short distance whenever it likes.^ 



We can now proceed to consider the different modes of aerial 

 locomotion employed by birds, and in doing this it will be con- 

 venient to take first (I.) gliding flight, as this is the simplest. Its 

 commonest form is when a bird, having acquired a certain velocity, 

 intermits the flapping of its wings, and, with them and its tail 

 extended, floats or skims forward in the air. This is especially 

 common with such birds as Herons, Storks, Buzzards, Gulls, and 

 others Avith a relatively large sail -area. The required velocity 

 of movement may be obtained by descending ; as for example, 

 when a Pigeon flies from the roof of a house to the ground below 

 it visually glides or skims down. The movement is, from its 

 nature, a temporary one, involving as it does a loss either in 

 vertical height or in relative velocity, i.e. in motion through the 

 air. The direction of the trajectory may be up as well as down. 

 In the case of a Falcon, which swoops down on its quarry, the 

 altitude which is lost may, in case of its missing its prey, be in 

 great part recovered by gliding upwards on extended wings. As 

 was long ago known by falconers, these swoops involve little 

 exertion on the part of the birds, which do not pant as they do 

 after severe effort. Here the velocity acquired by descending with 

 the wings close to the body is used to raise the bird again. If 

 a bird moving in a horizontal line seeks to glide upwards, the 

 height to which it will be able to rise Avill correspond Avith the 

 square of the initial velocity. 



We can get some idea of the efficacy of this mode of flight by 

 observations on the rapidity of motion of birds which descend in 

 gliding for some distance obliquely in a straight line. This has 

 been done by Bretonnifere,^ who found that Storks Avere in the habit 



^ This illustration is given because, besides being of historical interest, it is 

 clear and easily comprehensible. It was accurate enough for the time when 

 written, but is now open to criticism from several points. For instance, the 

 sail-area of the Rook ought to include an allowance for the area of the body 

 measured in the plane of flight as well as the area of the wings. The support- 

 ing force, i.e. the thrust upwards on the Rook's wings and body, can be suffi- 

 ciently accurately expressed by the formula 



j^j^j.gy,pjrsinacosa 

 4 + IT sin a 

 where M is the force on the plane at right angles to the direction of motion, 

 and the other letters are as before. 



By some curious oversight, Cayley stated that an increase of speed from 23 '6 

 ft. to 27 "3 ft. per second will give an increase in the supporting force sufficient to 

 raise it from '4 of a pound to one pound. It ought to be 37 "3 instead of 

 27-3. 



" "Etudes sur le vol plane," L'Aironaute, juillet 1889. Reproduced by 

 Marey, Vol des Oiseaux, p. 296. 



