While No. 5 was thus under construction, new and somewhat larger engines had been built for No. 4, the work on them having been begun in January. The cylinders of these engines, which are more fully described in connection with Aerodrome No. 6, were 2.8 cm. in diameter, with a 5 cm. stroke, each cylinder thus having a capacity of 30.8 cu. cm., which was an increase of 36 per cent over that of the old brass cylinder engines, which had previously been used on No. 4. On April 28, under a pressure of 70 pounds, these engines drove the two 60 cm. propellers at a rate of 900 R. P. M., and lifted on the pendulum nearly 40 per cent of the total flying weight of Aerodrome No. 4, which was now approximately 5 kilos. A trial was made at Quantico in the latter part of May, which is described in Chapter IX. It is only necessary to mention in this connection that there was a great deal of trouble experienced with the alcohol aeolipile, the flame being extinguished in the moderate wind to which the [p067] aerodrome was subjected while preparations for the launch were being made. Moreover the flame was so nearly invisible in the sunlight that it was uncertain whether it was burning in the critical instants just before the launch, when doubt might be fatal. These conditions resulted in a final decision in favor of gasoline, on account of its greater inflammability, and in the provision of such hull covering that the fires could be lighted and maintained in a breeze. In June, I tried a modification of the burner, in which the gasoline was delivered under the pressure of air to the evaporating coil. In the first trial steam was raised to a final pressure of about 70 pounds, and a run of 45 seconds was secured under a pressure of 40 pounds in the gasoline tank, which was thought to be altogether too high; for, at the end of the run, the whole apparatus was enveloped in flames, because of the gasoline that was projected through the burner-tips.