30 
BIRD-LIFE. 
Flight, the most important of all the bird’s move¬ 
ments, can here be discussed only in general terms. The 
feathers of the wing are laid like the tiles on a roof, one 
overlapping the other; they are arched, and give a 
convex form to the upper surface of the wing. By raising 
the pinion the air is allowed to pass between the feathers, 
while in its descent they offer an insuperable resistance. 
This partially explains the fact, that a bird always either 
rises with each stroke of the wing, or keeps at the same 
level, and is never in the least depressed by it. The 
forward movement is attributable to the fact, that all 
strokes of the pinion do not fall in a perpendicular 
direction, but slope obliquely downwards from the front 
towards the back. By this means the wing is so canted 
as not to present its surface horizontally to the air on 
rising, but rather to cut through with its edge; more¬ 
over, the pressure of the pinion downwards is quite equal 
to four times that of the upward stroke: this is proved 
by a simple examination of the respective muscles. The 
tail serves as a rudder, and is bent somewhat in an 
upward direction while the bird is rising, and in a 
downward one in its descent; in turning it takes a 
slanting position. When soaring or circling, the tail 
alone directs the course of flight, while the position 
of the apparently motionless wings determines the 
greater or less rapidity of the same. The relative 
rapidity and the nature of the flight is in perfect harmony, 
with the formation of the wing and the construction of 
the feathers. All birds possessing long, narrow, sharp- 
pointed wings and close smooth plumage are rapid flyers 
in a straight line, though unable to diverge from their 
course with the same quickness as birds with shorter and 
rounder wings. With the faster flyers the wings over- 
