THEORY OF FLIGHT. 275 



the resistance which they meet with in the air has yet to be 

 determined. Our experiments on this subject are still being 

 pursued. When once we have these two elements, the mea- 

 sure of work will be obtained from the resistance which is 

 presented to the wing by the air at every instant, multiplied 

 by the distance passed over. This will give us the measure 

 of work bi'ought to bear upon the air. 



For its horizontal advance the bird will be obliged only to 

 furnish the quantity of work equivalent to the resistance 

 presented by the air in front of it, multiplied by the distance 

 passed through. A part of this resistance, namely, that 

 which is applied to the lower surface of the wing, is utilised 

 to sustain the bird, by the kind of action which we have com- 

 pared to that of a child's kite. 



It appears that this action is of primary importance in the 

 flight of the bird. In fact, among the researches on the 

 resistance of the air there is one which we owe to Mons. de 

 Louvrie, which seems to prove that if the wing make a very 

 small angle with the horizon, nearly all the work obtained 

 from the dynamic energy of the bird is employed to sustain 

 it ; according to this writer, an angle of 6 30' would be the 

 most favourable to the utilisation of its energy. The im- 

 portant part played by the gliding of the wing upon the air 

 seems also proved by the shape of that organ. The wing 

 being alternately active when it strikes the air, and passive 

 when it glides through it, is not, in all its parts, equally 

 adapted to this double function. 



When a surface strikes the air, it must move with rapidity 

 in order to find resistance. Thus the wing, turning around 

 the point by which it is attached to the body, shows unequal 

 and gradually-increasing velocity in different points according 

 as they are nearer to the body, so that being almost nothing 

 at the point of attachment of the wing, the velocity will be 

 very great at the free end. 



Let us imagine the wing of an insect as large at the base 

 as at the extremity ; this size would be useless in the part 

 nearest to the body, for the wing, at this point, has not suffi- 

 cient rapidity to strike the air with effect. Thus we find, in 

 the greater part of insects, the wing reduced to a strong 



