we find that we have 363·63 horse-power in actual effect delivered on the screws of the machine, which shows that there is rather less than 22 per cent. loss in the engines, due to cutting off before the end of the stroke, to back pressure, and to friction. The actual power applied to the machine being 363·63 horse-power, it is interesting to know what becomes of it. When the machine has advanced 40 miles (which it would do in an hour), the screws have travelled 68·1 miles (375 × 16 × 60/5,280) = 68·1; therefore, 150 horse-power is wasted in slip, and 213·63 horse-power consumed in driving the machine through the air. Now, as the planes are set at an angle of 1 in 8, the power actually used in lifting the machine is 133·33, and the loss in driving the body of the machine, its framework and wires through the air is 90·30 horse-power. Power lost in screw slip, 150 H.P. „ „ driving machinery and framework, 80·30 „ „ actually consumed in lifting the machine, 133·33 „ ------ Total power delivered by the engines, 363·63 „ THE ADVANTAGES AND DISADVANTAGES OF VERY NARROW PLANES. [Illustration: Fig. 86.--The path that the air has to take in passing between superposed aeroplanes in close proximity to each other. By this arrangement the drift is considerably increased.] My experiments have demonstrated that relatively narrow aeroplanes lift more per square foot than very wide ones; but as an aeroplane, no matter how narrow it may be, must of necessity have some thickness, it is not advantageous to place them too near together. Suppose that aeroplanes should be made 1/4-inch thick, and be superposed 3 inches apart--that is, at a pitch of 3 inches--one-twelfth part of the whole space through which these planes would have to be driven would be occupied by the planes themselves, and eleven-twelfths would be air space (Fig. 86). If a group of planes thus mounted should be driven through the air at the rate of 36 miles an hour,[12] the air would have to be driven forward at the rate of 3 miles an hour, or else it would have to be compressed, or spun out, and pass between the spaces at a speed of 39 miles an hour. As a matter of fact, however, the difference in pressure is so very small that practically no atmospheric compression takes place. The air, therefore, is driven forward at the rate of 3 miles an hour, and this consumes a great deal of power; in fact, so much that there is a decided disadvantage in using narrow planes thus arranged. [12] The arrows in the accompanying drawings show the direction of the air currents, the experiments having been made with stationary planes in a moving current of air.