268 FLIGHT 
remiges, shewn by the short lines in the figure, taken in a line 
parallel to the trajectory of the bird, is inclined at an angle with 
A 
the line of flight, so that their under surface looks downwards 
and backwards. The exact angle is not known, but it is certainly a 
small one, and _ probably 
varies with the velocity of 
the bird’s motion. Having 
made the down and forward 
stroke, the wing moves 
backward, being still ex- 
tended, and_ still inclined 
slightly backwards. The 
diagram indicating a bird 
flying from left to right, 
though not to be taken as 
more than approximately 
exact, gives an idea of what 
is known regarding the tra- 
jectory of the wing and the inclinations of its plane Ww ith the axis 
of flight. The down stroke, it should be added, takes a longer time 
than the up stroke. In making the latter, as can be seen from the 
figure (Fig. 8), the wing passes at first backwards, and then, becom- 
ing partially flexed with a w hiplike action, it rises upwards, the plane 
of the wing being altered during the up stroke, so that it looks 
down and for Ww ard. If the bird he flying fairly rapidly through the 
air, the up stroke is mainly a passive movement, the bird continuing 
to rest on the wings, and the velocity of the forward motion of its 
body diminishing, to be increased again with the next down stroke. 
At starting, and before the bird has acquired velocity, the up 
stroke is an active one, and the primary feathers can be proved 
to separate from one another, facilitating thereby the movement 
by reducing the resistance offered by the air to the back of the 
wing. The above description applies mainly to the Pigeon and 
Fia. 7. (From Marey.) 
