PRAGA: 267 
Il. Flight by actwe strokes of the wings. It is very much 
easier for a bird to sustain itself in the air by active flight, if it 
have a certain initial relative velocity, than if it seek to begin flying 
from a position of relative rest or want of motion. This leads all 
birds to rise from the ground with their heads towards the wind. 
If there be not wind enough, the bird seeks to gain initial velocity 
by running or springing, or both. In the case of aquatic birds 
the velocity is obtained in part by striking the water with their 
wing-tips during the first few strokes. In “hovering,” which is 
only possible for very powerful flyers, the great exertion can be 
seen from the rapidity with which the wing-strokes follow one 
another. Birds with short legs and long wings, as the Condor and 
Albatros, cannot, in the absence of wind, rise at all from the 
ground unless they have room to run, and Condors can be easily 
caught by tempting them with food within a narrow enclosure. 
Even the strong-flying Pigeon, after being made to rise from the 
ground and fly a short distance for five or six times in succession, 
refuses to rise again, and remains on the ground panting with open 
beak. Marey, who observed this in his own Pigeons, calculates 
that the energy expended per second in a Pigeon when taking 
flight is five times as great as when it has acquired a certain velo- 
city. The bird at starting makes rapid strokes with its wings, 
which move through a large angle,—in the case of the Pigeon 
striking one another above the back at the end of the up stroke, 
and nearly touching with their tips at the end of the down stroke. 
When velocity has been acquired, the flaps are slower, and the 
angle which they describe round the shoulder-joint is a very much 
smaller one. The reason why so much more energy is required to 
fly when they have little or no initial velocity relative to the air is 
due to the increased support afforded by the air if the wing-surface 
which strikes it be at the same time travelling through the air in a 
line more or less parallel to its surface. ‘This gain in resistance to 
the wing-stroke increases, as we have already pointed out, with the 
square of the velocity of the wing in a direction parallel with the 
axis of flight. 
‘With regard to the manner in which a bird uses its wings 
during flight we are indebted chiefly to Marey, who employed 
much more exact methods of observation than had previously been 
made use of. As can be seen from the annexed figure (Fig. 6), 
which shews a Gull photographed at successive intervals of one- 
fiftieth of a second, the wings during the down stroke move forward 
as well as downward. It can be shewn that the outer end of the 
humerus describes a kind of ellipse(Fig. 7) round a straight line passing 
through the shoulder-joints, the long axis of which ellipse is inclined 
slightly downwards from the horizontal. During the down stroke, 
which is made along the front half of the ellipse, the surface of the 
