As an example of this, let us take a speed of ten miles an hour, which means a pressure of one- half pound. Double this speed, and we have 20 miles. Multiplying one-half pound by 4, the result is 2 pounds. Again, double 20, which means 40 miles, and multiplying 2 by 4, the result is 8. Doubling forty is eighty miles an hour, and again multiplying 8 by 4, we have 32 as the pounds pressure at a speed of 80 miles an hour. The anemometer, however, is constant in its speed. If the pointer should turn once a second at 10 miles an hour, it would turn twice at 20 miles an hour, and four times a second at 40 miles an hour. GYROSCOPIC BALANCE.--Some advance has been made in the use of the gyroscope for the purpose of giving lateral stability to an aeroplane. While the best of such devices is at best a makeshift, it is well to understand the principle on which they operate, and to get an understanding how they are applied. THE PRINCIPLE INVOLVED.--The only thing known about the gyroscope is, that it objects to changing the plane of its rotation. This statement must be taken with some allowance, however, as, when left free to move, it will change in one direction. To explain this without being too technical, examine Fig. 63, which shows a gyroscopic top, one end of the rim A, which supports the rotating wheel B, having a projecting finger C, that is mounted on a pin-point on the upper end of the pedestal D. _Fig. 63. The Gyroscope._ When the wheel B is set in rotation it will maintain itself so that its axis E is horizontal, or at any other angle that the top is placed in when the wheel is spun. If it is set so the axis is horizontal the wheel B will rotate on a vertical plane, and it forcibly objects to any attempt to make it turn except in the direction indicated by the curved arrows F. The wheel B will cause the axis E to swing around on a horizontal plane, and this turning movement is always in a certain direction in relation to the turn of the wheel B, and it is obvious, therefore, that to make a gyroscope that will not move, or swing around an axis, the placing of two such wheels side by side, and rotated in opposite directions, will maintain them in a fixed position; this can also be accomplished by so mounting the two that one rotates on a plane at right angles to the other. _Fig. 64. Application of the Gyroscope._ THE APPLICATION OF THE GYROSCOPE.--Without in any manner showing the structural details of the device, in its application to a flying machine, except in so far as it may be necessary to explain its operation, we refer to Fig. 64, which assumes that A represents the frame of the aeroplane, and B a frame for holding the gyroscopic wheel C, the latter being mounted so it rotates on a horizontal plane, and the frame B being hinged fore and aft, so that it is free to swing to the right or to the left.