FLIGHT 269 
Gull, on which the observations have for the most part been made, 
but there is at present no reason for supposing that the active 
flight of other birds differs from that described in any essential 
Fic. 8.—PHOTOGRAPHIC TRAJECTORY OF THE TIP OF A Crow’s Winc. The little arrows shew 
(Fig. ic inverted) the direction of the tip’s movement. (From Marey.) 
particulars, though there is still a good deal to be learned about the 
mechanics of active flight, and data sufficient, for example, to enable 
us to calculate the work done by a bird in flying through the air 
are still wanting. We have, however, in the above given only a 
very incomplete sketch of what has already been learned on the 
subject. 
III. We now come to the interesting subject of soaring or sailing. 
The typical soaring birds come under Miillenhoff’s ? “ Vulture-type,” 
whose sail-area in relation to the total weight of the bird is a large 
one. In this category come such birds as the larger Corvidx, viz. 
the Raven and Crow, the Falcons and Vultures, the Owls, Pelicans, 
and Storks. Those of them which can soar are mostly large birds, 
with a relatively large wing-area, and few of them are commonly 
visible in this country, which is possibly one reason why the whole 
subject of soaring flight is still so obscure. 
The main characteristics of the soaring flight of such birds 
have, however, been recorded by a number of trustworthy 
observers, and are no longer seriously disputed. A certain 
amount of wind appears to be essential, soaring flight not being 
observed in a dead calm. Observers seem to agree also in this, 
that the soaring bird, with motionless outstretched wings (having 
raised itself some distance from the ground or sea by active wing- 
strokes), describes in its flight, curves or circles which lead it to 
alternately sail up the wind and down the wind. It describes 
wide curves, and loses in vertical position while it is directed down 
1 Tom, cit, p. 425. 
