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 with 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 usually 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, 7e. 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 will correspond with 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 Bretonniére,? who found that Storks were in the habit 
1 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, ¢.e. the thrust upwards on the Rook’s wings and body, can be sufli- 
ciently accurately expressed by the formula 
3 opm Sin a cosa 
oS LSS raring 
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°38. 
2 «“¥tudes sur le vol plané,” ZL’ Aéronaute, juillet 1889. Reproduced by 
Marey, Vol des Oiseaux, p. 296. 
