the planes, and its body, or fuselage, was covered in and streamlined, so as to offer the least possible resistance to the air. A later difficulty caused by the forward position of the airscrew had nothing to do with flying. When the war came, and machine-guns were mounted on aeroplanes, a clear field was needed for forward firing. This difficulty was ultimately met by the invention of a synchronizing gear, which timed the bullets between the strokes of the airscrew-blades. In all but a few types of machine the airscrew is now retained in the forward position. The debate between monoplane and biplane is not yet concluded; the biplane holds its own because with the same area of supporting surface it is much stronger and more compact than the monoplane. Instead of wing-warping, which puts a strain on the supporting surfaces and is liable to distort them, the French (to whom Bleriot is believed to have shown the way) introduced ailerons, that is, small subsidiary hinged planes attached to the extremities of the wings. By controlling these, one up and the other down, in conjunction with the rudder, the pilot can preserve his lateral balance, and turn the machine to right or left. Later on, these ailerons, when they were borrowed by the Voisin and Farman biplanes, were not fitted to the extremities of the planes, but became hinged flaps forming the extreme section of the trailing edge; and this position they have kept in all modern aeroplanes. An even greater advance was made by the French school in its device for the control of the machine. The machine which Wilbur Wright flew in France was controlled by two upright levers, grasped by the pilot, one in either hand. The left-hand lever moved only backwards and forwards; it controlled the elevator and directed the machine upwards or downwards. The right-hand lever controlled the rudder and the warping of the wings. By moving it backwards or forwards the pilot turned the machine to right or left; by moving it sideways he warped the wings. There is nothing instinctive or natural in these correspondences; the backward and forward movement which in one lever spells up and down in the other spells right and left. It is a testimony to the extraordinary cool-headed skill of the Wrights, and to their endless practice and perseverance, that they were able to fly such a machine in safety, and to outfly their rivals. The French school centralized the control in a single lever with a universal joint attachment at the lower end. The movements of this lever in any direction produced the effects that would instinctively be expected; a backward or forward movement turned the machine upwards or downwards, a sideways movement raised one wing or the other so as to bank the machine or to bring it to a level position again. The vertical rudder was controlled either by a wheel attached to this central lever, or by the pressure of the pilot's feet on a horizontal bar.