The twisting is brought about by a pull on the rope 3, which is attached at _d_ and _c_, and passes through pulleys at _g_ and _h_. When the rope is pulled toward the left the right end is tightened and slack is paid out at the left end. This pulls down the corner _d_, and raises _e_. The corner _a_ is raised by the post which connects _a_ and _e_. The rope 4, passing from _a_ to _b_ through pulleys at _m_ and _n_, is thus drawn toward _a_ and pulls down the corner _b_. Thus _a_ is raised and _b_ is lowered. At the same time rope 4 turns the rear rudder to the left, as shown by the dotted lines, thus forcing the side _R_ against the wind. Of course, if the left side of the machine starts to fall, the rope 3 is pulled toward the right, and all the movements take place in the opposite direction. The ropes are connected to a lever, by which the operator controls the warping of the planes. These movements are possible because the joints are all universal, permitting movement in any direction. In whatever position the planes may be set, they are held perfectly rigid by the two ropes, together with others not shown in the figure. The machine is guided up or down by the front horizontal rudder. When the aeroplane swings round a curve the outer wing is raised because it moves faster than the inner wing, and therefore has greater lifting force. Thus the aeroplane banks its own curves. The Wright flying-machine is called a biplane because it has two principal planes, one above the other. A number of successful flying-machines have been built with only one plane, and these are called monoplanes. A monoplane that early became famous is that of Blériot (Fig. 98). The Blériot monoplane was the first flying-machine to cross the English Channel. This machine is controlled by a single lever mounted with a ball-and-socket coupling, so that it can move in any direction. When on the ground it is supported by three wheels like bicycle wheels, so that it does not require a track for starting, but can start anywhere from level ground. The Wright and the Blériot represent the two leading types of early successful flying-machines. [Illustration: Copyright by M. Brauger, Paris FIG. 98--THE BLÉRIOT MONOPLANE] Submarines Successful navigation beneath the surface of the water, though not carried to the extent imagined by Jules Verne, was a reality at the beginning of the twentieth century. Instead of twenty thousand leagues under the sea, less than a hundred leagues had been accomplished, but no one can foretell what the future may have in store.