82 Froceedings of Royal Society of Edinburgh. [sess. 
may support itself without direct expenditure of energy for the 
purpose. 
Sufficient has been proved to show the advantage which must 
accrue to one that skilfully negotiates a gust. But the resources of 
a sagacious bird are not exhausted. 
When there are no gusts, and the wind is steady, or in the case 
under consideration, when the variations of its velocity do not 
exceed 23 ‘3 feet per second, if the bird can, by utilising part of its 
acquired velocity, by swooping down, or making a flap or two, get 
into another current, or anywhere where the air is comparatively 
still, it will be in the same position as occurred at the lull, which 
we had sujDposed in the previous instance to follow the gust. 
Actual observation, in the light of this theory, makes it quite 
evident they do make use of such tactics, and that it aids them 
considerably in sustaining their flight. 
The author has observed Gheel ascend the spur of a mountain 
some hundreds of feet by following manoeuvres which plainly 
illustrate the working of the above principles. 
Take AB, fig. 2, for the axis of the spur. The curved line 
shows the course of the bird. Flying towards the spur, it ceases 
to use its wings at a and rises by virtue of its attained velocity to 
&, where it meets the wind coming over the edge of the hill, and 
is thus driven quickly to e ; here it finds the air stagnant. Sheltered 
by the rising slope of the spur, it drops a little to reach still quieter 
air, and utilises its thereby further-increased velocity to bring it 
back again towards the hill, the operations being continued in- 
definitely. 
In the instance observed by the author, the bird found it neces- 
sary to make from' four to six flaps of its wings at points c, e, &c. 
It may be also remarked that most birds in a high wind, when 
they pass such an object as a church steeple, for instance, will make 
use of the changed velocity of wind on its lee side to raise them- 
selves perceptibly in their flight. 
In such ways the wind, to us a hitherto insuperable obstacle to 
practical aeronautics, is turned to account by unintelligent members 
of the animal creation. They trade, as it were, between two regions 
of air, bringing energy acquired in one to make use of in another. 
The difficulty of adopting the same practice with flying machines 
