“To be sure it did; because, by that operation, you called a new force into action; which I have represented in the diagram by the line S T. The kite was therefore under the influence of the two forces O A and S T, and, since these are in the direction of the two sides of a parallelogram, it would not obey either, but ascend through O B, its diagonal.”
“Notwithstanding Mr. Twaddleton’s doubts upon the subject,” said Tom, “I am sure that I perfectly understand your explanation; and I think I may also answer for my sister: but you have not yet told us any thing about the tail; I suppose, however, that it acts like the rudder of a ship, or the tail of a bird.”
“Before I answer that question, let me inform you how the tail of a bird differs, in its action and uses, from the rudder of a ship. In the first place, the rudder is so fixed that it can but move in one horizontal plane, and can therefore only turn the vessel to the right or left, which, indeed, is all that is required;[(36)] but the tail of the bird, in addition to this motion, can be placed in a diagonal direction, and when expanded will offer a considerable surface to the air, so as to fulfil some of the offices of a third wing. Have you never watched the manœuvres of the rook, as he gambols through the air? After flying in the ordinary way, you will observe his wings at rest, and that he glides along apparently without the least exertion in his descent. In this case, his expanded wings act as a parachute; then, again, you will observe him wheeling round, a manœuvre which is partly produced by the oblique position of his tail, and which is readily explained upon the principle of the resolution of forces I have just described with reference to the action of the wind upon the surface of the kite.[(37)] I ought also to state, that the tail serves to poise the body of the bird.”
“Does the bird, then, never use its wings for the purpose of directing its course?” asked Louisa.
“Undoubtedly it does,” answered her father; “the tail is only to be considered as a supplementary organ: it is by means of the wings that it generally directs its course, for it is evident, that it can easily turn, either to the right or left, by flapping the opposite wing with increased force, just as a boat is turned about to the right, by a brisk application of the left oar. In like manner the irregular flight of the butterfly, now up and now down, now to the right and now to the left, is no doubt effected by the wings striking the air one after the other, or perhaps with an alternate and unequal force. The object of such an action is obviously to baffle the pursuit of birds which fly in a right line, whereas you see the butterfly does just the contrary.”[(38)]
“How very wonderful,” said Louisa, “is the action of the wings of insects. I have often watched them during their flight, and their rapidity is such as to surpass the power of vision.”
“I shall have occasion to advert to that subject hereafter,” said Mr. Seymour; “at present, I shall only observe, that a gnat’s wing, in its ordinary flight, beats many hundred times in a second.”
“But you have not yet answered Tom’s question,” said the vicar. “Of what use is the tail of the kite? Does it assist its ascent, or is it merely an appendage of ornament?”
“In the first place, it keeps the head of the kite to the wind; and in the next, it lowers its centre of gravity, and throws it towards its extremity, which not only prevents the chance of the machine being upset in the air, but so poises and regulates the position of the kite as to maintain the angle which it is necessary for the string to make with the surface.”
Mr. Twaddleton here enquired what might be the most advantageous angle for the kite to form with the horizon, in order that the paper machine should rise to the greatest altitude.