First we must recall the "A B C facts" we learned about wing construction. A wing gains lifting power from two sources: the upward pressure of the air current underneath it, and the force of the vacuum above it which is created by the arch of the wing. If a wing is only slightly arched it can move _forward_ through the air more swiftly, but it will not have the _lifting power_ of the high arched wing. This is the reason that an airplane which must be a weight carrier cannot be as fast in flight as the "speed scout," which has only its pilot and a machine gun to carry. The "speed scout" is always a small machine, usually a single-seater, with a gun in front that fires over or through the propeller. In the early part of the Great War it was most often a monoplane, but the smaller biplane took its place, because, with practically the same speed, it combines greater stability. The planes of the speed scout are very flat as compared with those of the reconnaissance craft. This airplane must carry machine guns, photography apparatus, radio, and a pilot, an observer, and often a gunner. Its wings must therefore be arched to give it lifting power, but at the same time it becomes a much slower flying machine than its smaller sister. Lifting power of a wing can of course be increased _up to a certain point_ by increasing the wing area, so that a greater air pressure is created below. Beyond that certain point the machine would become unwieldy and would lose its balancing properties. Yet this idea has been put into practise in building the latest types of aerial dreadnaughts used for reconnaissance. These airplanes have gained their lifting power partly by increasing the wing spread and partly by arching the wing. Thus a wing has been secured which offers the minimum resistance to forward motion through the air, together with the maximum weight carrying ability. Biplanes of this type are by far the most popular of those designed for general service, for they combine speed, climbing ability, and lifting power,--thanks to their strong armament they can defend themselves or run away quickly as the situation demands. But there is one other method which has not yet been mentioned of increasing the lifting power of an airplane. It is simply to add a third wing. When we have made the wings of the biplane as large as we dare, and have curved them to make them weight-bearers, if the resulting machine is still not strong enough to carry as many tons of explosives as we desire, there is only one thing left to do and that is to add a third wing. Thus the triplane made its appearance in answer to the call for planes which could carry vast cargoes of explosives and fuel for journeys of many hours over the enemy's country. The huge night bombing machine of the present time is almost always of the triplane type. CHAPTER V SOME OF THE PROBLEMS THE INVENTORS HAD TO SOLVE