Aerodrome No. 3 (Plate 10) was an attempt to obtain better conditions than had existed in the preceding model without any radical change except that of moving the cross frame, which carried the engines and propellers, nearer the front of the machine. Instead of the oscillatory engines used up to this time, two stationary cylinder engines, each 2.4 cm. in diameter and 4 cm. stroke, having a combined capacity of 36 cu. cm. without cut-off were employed for driving the propellers. The engines, though occasionally run in trials with steam from a stationary boiler, were intended to be actuated either by compressed air or carbonic-acid gas contained in a reservoir which was not actually constructed, but whose weight was provisionally estimated at 1 kilogramme. The weight of the aerodrome without this reservoir was but 1050 grammes, including the estimated weight of the sustaining surfaces, which consisted principally of two wings, each about 1 metre in length by 30 cm. in breadth and which were in fact so slight in their construction, that it is now certain that they could not have retained their shape in actual flight. The only trials made with this aerodrome, then, were in the shop, of which it is sufficient to cite those of November 22, 1892, when under a pressure of 30 pounds, the maximum which the engines would bear, two 50 cm. propellers were driven at 900 revolutions per minute, with an estimated horse-power of 0.07, about 35 per cent of the weight of the whole machine being lifted. This was a much more encouraging result than any which had preceded, and indicated that it was possible to make an actual flight with the aerodrome if the boilers could be ignored, the best result having been obtained only with carbonic acid supplied without limit from a neighboring ample reservoir. This aerodrome was also tested while mounted upon a whirling-arm and allowed to operate during its advance through the air. The conclusion reached with it at the close of 1892, after a large part of the year passed in experiments with carbonic-acid gas and compressed air, was that it was necessary to revert to steam, and that whatever difficulties lay in the way, some means must be found of getting sufficient power without the weight which had proved prohibitory in No. 0. With this chapter, then, and with the end of the year 1892, I close this very brief account of between one and two years of fruitless experiment in the construction of models supplied with various motors, subsequent to and on a larger scale indeed than the toy-like ones of india rubber, but not even so efficient as those had been, since they had never procured a single actual flight. [p041] CHAPTER V ON SUSTAINING SURFACES The following general considerations may conveniently precede the particular description of the balancing of the aerodrome.