No type of aeroplane is more familiar in America than the Curtiss biplane. By long experimentation, this machine has been developed for practical use; and is now used for military purposes in Russia, Japan, Italy, Germany, France, and the United States. The machine is of the general type known as "biplane," in which there are two sets of wings, or surfaces, one being directly above the other. This type of machine seems to be the most favoured by Americans, for it not only allows of a greater spread of lifting surface for a given width of plane than in the monoplane, or single-wing type, but also it is much stronger than other machines of the same weight, as its design permits of a system of bridge-trussing known as the "Pratt Truss." In the Curtiss machine this feature is especially pronounced, because of the greater safety which rigid planes have when compared with the flexible wings. The woodwork of these aeroplanes is entirely of selected spruce and ash, all the posts, beams, and ribs being laminated. The propeller is a particularly difficult piece of laminated work, being built up of from twelve to eighteen layers of thinly cut wood, while the upright posts of the central section are made up of ash and spruce, the heavier and more flexible wood forming the core. A feature of strength is to be found in the double trussing which is placed in all of the vital parts of the aeroplane, where the greatest strength is required. All this trussing is made with a cable of galvanised steel wire tested to withstand a pulling strain of nearly half a ton. Transportation and military use have been especially considered in the construction of the planes. The upper and lower planes are made up of interchangeable panels, which are so joined together that the machine is easily assembled and taken apart and may be transported compactly in two flat boxes which scarcely make one full wagon load, as indicated in an illustration in this book. The wing-panels are made up with a light and strong wooden framework covered with cloth especially made and treated with a rubber coating for the purpose. The curved ribs are laminated also and the panels held together by a system of trussing which gives them great strength. These panels are covered both top and bottom.