As a test of the durability and the capabilities of aircraft, flying under war conditions cannot be equaled, for various reasons. Firstly, maneuvers, which in times of peace would be considered risky to life and thus avoided, must be endured daily by pilots flying over the battle area. Flying under shell-fire frequently necessitates maneuvers, entirely unaccounted for by the constructors of the machine, which put a very great strain on the framework, wings, struts, etc. To compensate for such strain, every wire, strut, and part of the framework is constructed of a strength at least eight times greater than that of the actual strength required. Thus the weak points of the machine are discovered, also the centers at which the greatest strain takes place. FUTURE TYPES OF CRAFT The shape and general build of the aeroplane has not thus far changed materially from the original models of Orville and Wilbur Wright, save that the majority of the modern machines are tractors (_i. e._ with the engine in front), whereas the older types were “pushers” (with engine at the rear). The new principle has naturally both advantage and disadvantage. With the tractor engine, the machine has a great speed, and is able to climb at a much faster rate, but the inherent stability of the craft is seriously affected--by shifting the engine 80 per cent. of the total weight is moved from the center to the nose of the aeroplane. To compensate for this the wings have had to be extended, and this has added considerably to the weight in aggregate. But this evil has again been remedied, by bringing the extreme ends further to the rear, and slightly indenting each wing-tip: in a word, constructing the aeroplane more and more after the fashion of a bird in flight. Such is the peculiar working of the human mind, however, that when some new theory or substance is evolved, similar to the one in question, it is content to concentrate on the original formula, and develops that rather than apply the same principles to an entirely new formula. Thus, after some twelve years of flying, we have only four distinct types of craft: the balloon, the airship, the aeroplane, and the seaplane--the two former being very similar both in principle and shape, as also the two latter. Exception cannot be made for the “triplane,” for that machine, with three planes, has the same shape as the aeroplane. The principles of aero-statics, and aero-dynamics by no means confine the constructor to these two standard forms; and in the near future the aeroplane will be built on similar lines to the ocean-going liner, and the airship very much on the same principle.