"By a system of triangulation," he replied. "In a few days we may have an opportunity to use the system, and I will explain it then so you can quickly grasp the subject." The air speed indicator was another interesting device, but at that the lieutenant shook his head as though doubting its general value. "Don't you think it's accurate?" inquired Alfred of the officer. "Oh, yes; it is as accurate as it can be made. In fact all devices, which are used to indicate the speed of the wind, are nearly perfect. That is not the difficulty. When it is used on a flying machine several new things enter into the problem, which makes it difficult for the instrument to be truthful." "What is the main trouble?" asked Alfred. "Well, suppose we are starting a flight against a wind, which is moving at the rate of, say, ten miles an hour," answered the lieutenant. "While in flight we note that, going against the wind, the air speed is sixty miles. Now, if we happened to stop suddenly so we were poised in the air, the indicator would show the air speed to be ten miles, so that we were really going only fifty miles. If, now, the machine is turned so that we are going at right angles to the movement of the wind, although we may be going actually through the air at a speed of seventy miles an hour, the indicator does not register the air speed at all, because it is in such a position that the air does not affect it and the only indication we have is the speed of the air relative to the movement of the machine,--or, to put it in a better way, I might say that the instrument only registers, in reality, the speed at which the aeroplane goes through the air, and no more." "But is that the only way to tell how fast we are going?" asked Ralph. "The best test is to take the engine revolutions. A certain machine after it has been properly tested in a calm, and also in wind currents of different velocities, will show a record of speed with the engine running at certain revolutions. An experienced aviator can judge pretty well, by experience, what the speed of the wind is, and, knowing the propeller speed, and the direction he is going relative to the wind, he can make a fair estimate of the speed of the machine." CHAPTER X A THRILLING EXPERIENCE IN THE AIR It was with feelings of joy that the boys stepped out of their tent the following morning to learn that the lieutenant had decided on a trial spin in the forenoon, preparatory to the real start, which was to take place at two in the afternoon. The engine was turned over, and the machine held down while the usual hour's running test was going on. Everything seemed to be in order, the engine was stopped and another inspection made when the lieutenant gave the order to mount the machine.