262 FLIGHT 
distinguishes siz,—namely the Quail, Pheasant, Sparrow, Swallow, 
Vulture, and Gull. 
In considering birds from the point of view of their flying 
capacities, it should be kept in mind that their anatomical structure 
is such as to give them great stability in the air. The wings are 
attached to the highest part of the thorax, so that the centre of 
gravity of the bird is as low as possible beneath the centre of 
suspension on the wings, and also that the same object is assisted 
by the light organs, viz. the lungs and air-sacs, being placed high, 
while the heavy organs of digestion, and above all the heavy 
pectoral muscles, are placed as low as possible. ‘The shape of a 
bird’s body, moreover, is such that it offers little resistance to its 
passage through the air. 
We must here say something about the resistance offered by 
the air to the passage of a body through it. As Newton first 
shewed, this resistance increases with the syuare of the velocity of 
the body. It increases also directly with the sectional area at 
right angles with the axis of motion (the geometrical form of the 
body being similar), and this velocity and area multiplied by one 
another and by a numerical coefficient gives the resistance.’ 
Of the greatest importance for the flight of birds is the fact 
that the resistance offered to the motion of a flat body in a 
direction at right angles to the plane of its surface is very greatly 
increased if it be made to move at the same time through the air 
rapidly in a direction parallel with its surface. 
As a clear comprehension of this matter is important, an 
illustration may be given from Sir George Cayley.2 Supposing a 
flat surface, such as a piece of cardboard, with a superficial area of 
one foot and inclined at an angle of six degrees to the line of 
movement, be carried forwards horizontally, as in the case of the 
wings of a gliding bird, it is found that with a rapidity of 23°6 
feet per second the pressure perpendicular to its surface is ‘4 of a 
pound, while if the speed be increased to 27°3 feet per second, it is 
equal to one pound. A Rook, whose weight and wing-area were 
found by Cayley to be, roughly speaking, in the ratio of one pound 
to the square foot, would therefore be able to glide horizontally 
whenever it had a velocity of 27°3 feet. But a Rook usually flies 
1 Expressed more exactly, the resistance, R, offered by the air to the passage of 
pm sin? a 
4+ rsina 
numerical coefficient, S the surface, V the velocity, p thewensity of the air, 
and a the angle of the surface with the line of motion. It must be understood 
here that the resistance is to the forward movement of the surface, ¢.e. the 
resistance in the line of motion. 
2 “On Aérial Navigation.” Jowrnal of Natural Philosophy, Chemistry, and 
the Arts (Nicholson’s), xxiv. p. 164 (1809). 
a flat surface through it may be stated as R= KS V? 
; K being the 
