“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.” “It appears to me that flying is an easy process,” said Tom; “could we not contrive some sort of flapper, by which we might be able to rise into the air?” “Your opinion, my dear boy, is by no means singular; hundreds have entertained the same belief before you; and so confident was the famous Bishop Wilkins, that he declared it to be his conviction, that, in future ages, it will be as usual to hear a man call for his wings, as it is now to call for his boots.” “Yes,” said the vicar; “and if my memory is correct, William of Malmesbury, in his account of the Conquest of England by the Normans, mentions a Benedictine monk, by the name of Elmer, who having affixed wings to his hands and feet, ascended a lofty tower whence he took his flight, but he fell to the ground and broke both his legs.” “I do not see the difficulty,” exclaimed Tom.