“The weight of our bodies is so great, that we have not sufficient muscular strength to impart a blow to the air that shall be equal to it. Now are you satisfied?” said his father. “I am perfectly satisfied, if that be the case, that we can never hope to fly.” “The principle, however, which I have just explained,” observed Mr. Seymour, “although it will fail us in our attempts to construct wings, is nevertheless extensively applicable in mechanics. A vane or fly, for instance, by resisting the air as it spins round, becomes the regulator of machinery. The intervals between the strokes on the bell of a clock are thus regulated, and the fly, on this occasion, is so contrived that this interval may be altered, or the clock made to strike faster or slower, by presenting the arms of it more or less obliquely to the direction in which they move. The same kind of fly is the regulator used in musical boxes, as I will presently show you, and indeed in almost all mechanical toys. Let us now return to the subject of the kite; for, as yet, we have merely considered the effect of increasing the wind upon its surface; we have next to enquire how the wind operates in raising it into the air.--Do you not remember, when I adjusted the noose in the belly-band, I stated that much depended upon this part of the apparatus? You will, at once, perceive that it will influence the angle which the kite forms with the earth, and I am about to prove to you, that the theory of the kite’s ascent is materially connected with the value of this angle; but, in order to render my explanation intelligible, I have prepared a diagram, to which I am desirous of directing your attention. [Illustration: Diagram of a kite labeled with A, B, O, S, T, W, and Y.] “The kite here appears in the act of rising from the ground; the line W will represent the direction of the wind blowing upon it, all the currents of which we will suppose united in one; it is evident, from what has been already stated, that as it falls upon an oblique surface, it will be resolved into two forces, viz. into one parallel with it, and into another perpendicular to that surface; of which the force represented by the line Y will alone produce any effect, carrying the kite along the line O A, or in a direction parallel to itself; and you must have observed that this was the direction in which the kite was impelled, when you suffered it to rise, without checking its progress by the string.” “I remember that well,” said Tom; “and I also observed that, when I pulled my string, the kite immediately rose more perpendicularly.” “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.”