The earliest to attain to practical success was the B.E. type of machine. Every pilot who had his training in the early days of the war was familiar with this machine, though not every pilot knew that the initials are a monument to Louis Bleriot, who first flew the Channel. His achievement gave a great vogue to his monoplane, which was imitated by many designers; and when the factory produced a biplane fitted, like all monoplanes, with a tractor airscrew, in front of the machine, the biplane was called the Bleriot Experimental. The F.E. type is the Farman Experimental, a pusher biplane, which for a long time held its own by virtue of two advantages. The observer, being seated in the very prow of the machine, could fire a gun forward without being obstructed by the airscrew. This advantage disappeared after 1915, when, by the invention of synchronizing gears, which timed the bullets to pass between the revolving blades of the screw, tractor machines were enabled to fire directly ahead. But another advantage persisted. In night-flying, when the eyes are strained to pick up dim shapes in the dark, a clear field of vision is all-important, and the F.E. type of machine continued to be used in night raids throughout the war. The third type was the S.E., or Scouting Experimental. The fifth variant of this type, the S.E. 5, gained an enormous reputation in the war as a fighting machine, and indeed was preferred by some pilots to the best scout machines of private makers. A fourth factory machine, produced just before the war, and no less famous than the other three, was called the R.E., or Reconnaissance Experimental. It was the first almost completely stable machine. Stability is not of the first importance to a fighting scout, whose attention is concentrated on his own manoeuvres, but where a machine is used for observation, and the pilot must needs pay heed to all that is visible on the earth beneath, stability is essential. A perfectly stable machine maintains an even keel in varying gusts of wind. If it is tilted, it rights itself. If it is nose-dived, the pilot has only to let go of the control, and after a descent of some hundreds of feet it comes out of the dive and resumes its horizontal flight. The perfecting of this type of machine was achieved at the factory, and was the work of many minds. On the mathematical side the theory of stability was investigated by Mr. F. W. Lanchester, an authority on the theory of flight, and by Professor G. H. Bryan, a great pioneer, who in 1911 produced his book on _Stability in Aviation_. He had long been interested in the subject; his work, which is recognized as epoch-making, laid a sound mathematical basis for the theory of flight, and directed the work of others along the lines of fruitful experiment. The theoretical conclusions of Professor Bryan were reduced to a practical form by Mr. Leonard Bairstow and the members of the staff of the National Physical Laboratory, who put the doctrine to the proof of