“I suppose that you thus obtain more force from the wind.”
“Certainly: action and reaction are equal. By running, therefore, with your kite against the wind, you strike the air, and thus produce a reaction, which is equal to the force of the blow given to it. When the wind is high, and its action is not intercepted by surrounding objects, there cannot exist any necessity for such an expedient.”
“The principle is the same as that which enables the bird to rise into the air by flapping its wings,” observed the vicar.
“Unquestionably,” replied Mr. Seymour.
“Does the kite, then, rise in the air, from the same causes that enable a bird to fly?” asked Tom.
“We are not, at present, considering the ascent of the kite, but the advantage which is obtained by running with it: this latter, as the vicar has properly observed, undoubtedly depends upon the same principle as that which enables the bird to rise, by the motion of its wings, and which constitutes the third law of motion,[[43]] viz. that action and reaction are equal; that is to say, whenever one body exerts a force upon another, the second body opposes the first, with equal force, in an opposite direction. If, then, the bird strikes the air below it with a force which is equal to its weight, then must there be a reaction of the air, upwards, exactly equal to it; and the bird, being acted upon by two equal forces, in opposite directions, will, necessarily, rest between them.”
“That is clear enough; but the bird rises,” answered Tom.
“Because the force of the stroke is greater than the weight of the bird, and it therefore rises with the difference of these two forces; were the stroke less than its weight, then would it sink with the difference. Suppose, for example, a bird weighs twelve ounces, and it strikes the air with a force equal to sixteen, is it not clear that it must rise with a force equal to four? and is it not evident that, if it strikes the air with a force equal only to eight, that it must sink with a force equal to four?”
“So far I understand it perfectly; but I was thinking that, as the wing flaps up and down, what was gained by striking the air downwards must be counterbalanced when the bird raised her wing again, and thus struck the air in the contrary direction,” observed Tom.
“I give you no small degree of credit for that remark,” said his father; “for it is undoubtedly true that, if the flapping of the wings in flight were no more than the motion of the same surface upwards and downwards, the bird must lose as much by one motion as she could gain by the other; the skylark could never ascend by such an action, for, as you have so justly remarked, although the stroke upon the air by the under side of her wing would carry her up, the stroke from the upper side, when she raised her wing again, would bring her down; but, if you will attentively examine the structure of the wing, you will at once perceive, from its external convexity, the disposition, and more particularly the overlapping of its larger feathers, that when the wing is drawn up, its surface is contracted, and when let down fully expanded--or, in other words, that the feathers strike the air downwards with their flat side, but rise from the stroke slantwise, just as the rower in a boat, after having given the stroke, turns his oar so as only to present its edge, an operation which is termed feathering, from its resemblance to this very action of the wing in flight.”