TRANSACTIONS OF SECTION A, 120 
motion’ various theories have been proposed, and amongst these the most important 
are the three which suppose the seat of available energy to lie in— 
(1) Upwerd air-currents (Mr. Maxim). 
(2) Variations of the wind-velocity at different heights above the ground (Lord 
Rayleigh). 
(8) Variations of the wind-velocity from one instant to another, the wind 
habitually blowing in gusts separated by lulls (Dr. 8. P. Langley and others). 
Before proceeding further, another source of energy may be mentioned, namely, 
the presence of vortices, z.e., miniature whirlwinds or cyclones, in the atmosphere. 
Even on a perfectly calm day one of these little vortices may sometimes be seen 
travelling across a road, carrying up a funnel-shaped cloud of dust. According to 
mathematical theory, a vortex always consists of the same particles of fluid; and, 
even under the modified conditions which occur in nature, our experience of 
cyclones tells us that such vortices are remarkable for their persistency, and their 
motions are so regular that it would be easy for birds to take advantage of them, 
This would account for the fact that birds so often congregate in a certain spot 
when in sailing flight. 
Against the third hypothesis it has been objected— 
(i.) That to take advantage of every puff of wind in such a way as to be lifted 
up by it would be an extremely difficult feat of aérial gymnastics, whereas birds 
appear to circle in the air without requiring to exercise any particular alertness or 
agility. 
i (u.) That the variations in wind-velocity are not sufficient to sustain the weight 
of a bird in the air. 
In answer to the first objection, it is to be observed that if the bird’s centre of 
mass is slightly below the wing-surface—especially if the wings are slightly curved 
upwards—the action will be purely automatic. We may illustrate this point perhaps 
better by considering the parallel effect in the seeds of many composite plants 
(such as the common ‘ dandelion’), which are supported in the air by a parachute 
placed at some distance above them. Ifa sudden gust of wind blows upon such a 
seed, the parachute is set in motion more rapidly than the seed, causing the structure 
to heel over so as to receive the wind on the under surface of the parachute, and 
this lifts the seed. When the wind subsides, the greater inertia of the seed carries 
it on in front of the parachute, causing the latter to again present its under side to 
the air, which again lifts the seed. The more the seed is blown about, the more it 
rises in the air. 
This action would take place automatically in the same way in any body whose 
supporting parachute, aéroplane, or wing surface was slightly above its centre of 
mass. The height of the supporting surface should not be too great, otherwise 
the body would heel over too much, and would make so great an angle with the 
horizon that the lift would be considerably reduced. 
The effect evidently depends on the znertia of the body, and the lift could 
therefore be increased by increasing the body’s mass. But this would also increase 
the weight of the body in the same proportion, so that no advantage would be 
gained, 
The difficulty is overcome in the case of the sailing bird by the increased 
buoyancy which it is able to obtain from the air in consequence of the horizontal 
speed at which it travels, and herein, to my mind, lies the answer to the second 
objection. Dr. S. P. Langley! has found (1) that a horizontal plane under the 
action of gravity falls to the ground more slowly if it is travelling through the air 
_ with horizontal velocity than it would do if allowed to fall vertically, ard (2) that 
the horse-power required to support a body in horizontal flight by means of an 
aéroplane is less for high than for low speeds. Hence it readily follows that the 
bird’s forward motion causes it to fall through a smaller height between successive 
gusts of wind than it would do if it were at rest, and that when a side wind strikes 
the bird (7.e. a wind at right angles to the bird’s course), the lift is considerably 
increased in consequence of the bird’s forward velocity. 
1 Experiments on 4érodynamics. 
