“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. “If the wind be horizontal,” answered Mr. Seymour, “it is evident that the inclination of the kite’s surface ought to be the same as that which the rudder of a ship should make with the keel, in order that the vessel may be turned with the greatest facility; supposing the currents of water, which impel it, to have a direction parallel to the keel.” “And what ought that angle to be?” enquired the vicar. “_Fifty-four_ degrees, and _forty-four_ minutes,” replied Mr. Seymour; “and let me here remark,” continued he, “as we have already considered the philosophy of the flight of a bird, that its pinions are so set upon the body as to bring down the wings in an _oblique_ direction towards the tail; so that in their action upon the air, we have the same resolution of forces as in that of the wind upon the surface of the kite, by which the body of the bird is not only supported, or raised perpendicularly, but carried forward.”(39) Tom here interrupted the dialogue, by expressing a regret that he should have been provided with so small a quantity of cord. “I do not believe, my dear boy, that any advantage could be gained by an additional quantity of string,” said his father. “Is there, then, any reason why the kite should not ascend, even above the clouds, provided that my string were sufficiently long and strong?” “Yes; indeed is there a most unanswerable reason. Remember that the kite is made to rise by the operation of two forces; the one afforded by the wind, the other by the action of the string; now it is quite evident that, when the weight of the string, added to that of the kite itself, becomes equal to the force of the wind, acting upon the surface of the machine, a general balance, or equilibrium, of forces will be established, and the kite can no longer continue to ascend.” “Will it, then, remain stationary under these circumstances?” asked Louisa. “It must do so, unless the force of the wind should abate; for it is a proposition in mechanics, which I shall hereafter endeavour to demonstrate(40), that, if a body be acted upon by three forces, which are proportional to, and in the direction of, the three sides of a triangle, it will be kept at rest. The kite is exactly in this predicament, for its weight, the force of the wind, and the action of the string, fulfil these conditions, and consequently keep the kite stationary.”