262 FLIGHT 



distinguislies s.ix, — 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 wangs 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 square 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.^ 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 2 3 '6 

 feet per second the pressure perpendicular to its surface is "4 of a 

 pound, while if the speed be increased to 2 7 '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 2 7 '3 feet. But a Rook usually flies 



^ Expressed more exactly, the resistance, R, offered by the air to the passage of 



• 2 



a flat surface through it may be stated as R = K S V ^ -^ ; — ; K being the 



° •' 4 + TT sin a * 



numerical coefficient, S the surface, V the velocity, p the density 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 fonvard movement of the surface, i.e. the 



resistance in the line of motion. 



^ " On Aerial Navigation." Journal of Natural Philosophy, Chemistry, and 



the Arts (Nicholson's), xxiv. p. 164 (1809). 



