“I understand you, papa; the force of the wind, by striking the oblique feathers, is resolved into a perpendicular and parallel force, as you explained to us, when we considered the action of the wind upon the kite.” “Exactly; every oblique direction of a motion is the diagonal of a parallelogram, whose perpendicular and parallel directions are the two sides. Having settled this point, let us consider the second; viz. how it happens that the cork of the shuttlecock always presents itself to the battledoor.” “I should think,” said Tom, “that the cork points to the battledoor for the same reason that the weathercock always points to the wind.” “Admirably illustrated!” exclaimed his father; “the cork will always go foremost; because the air must exert a greater force over the lighter feathers, and therefore retard their progress. While we are upon this subject, I will introduce to your notice some contrivances which are indebted to this same principle for their operation. In the first place, there is the arrow; can you tell me, Louisa, the use of the feathers which are placed round its extremity?” “To make its head proceed foremost in the air, by rendering its other end lighter, and therefore more sensible to the resistance of the air,” replied Louisa. “Very well answered; that is, unquestionably, one of the objects of the wings of an arrow; but there is also another, that of _rifling_ it, or steadying its progressive motion, by causing it to revolve around its axis. If you will look at this arrow, you will perceive that the feathers are placed nearly, but not quite, in planes passing through it; if the feathers were exactly in this plane, the air could not strike against their surfaces when the arrow is in motion: but, since they are not perfectly straight, but always a little aslant, the air necessarily strikes them, as the arrow moves forward; by which force the feathers are turned round, and with them the arrow or reed; so that a motion is generated about its axis; and its velocity will increase with the obliquity of the feathers. You will therefore observe that, in order to enable the feathers to offer a necessary resistance to the air, they must possess a certain degree of stiffness or inflexibility. It was on this account that Roger Ascham,[46] and other skilful artists in the days of archery, preferred the feathers of a goose of two or three years old, especially such as drop of themselves, for pluming the arrow; and the importance, as well as the theory of this choice, is confirmed by a curious observation of Gervase Markham,[47] who says that ‘the peacock feather was sometimes used at the short butt; yet, seldom or ever, _did it keep the shaft either right or level_!’”(43) “That is intelligible enough,” said Tom, “the feather of the peacock must have been so flexible as to have yielded to the slightest breath of air; and now, as we are upon the subject of the arrow, do explain to us the action of the bow.”