The main planes of this first machine had a spread from tip to tip of 40 feet, and measured 6 feet 6 inches from the entering edge to the rear edge, a total area of 540 square feet. This will show how great is the spread of the main planes as compared to their length from front to rear. The two surfaces were set six feet apart, one directly above the other, while the elevating rudder was placed about ten feet in front of the machine on a flexible framework. This elevating rudder was composed of two parallel horizontal planes which together had an area of eighty square feet. The elevating planes could be moved up or down by the operator just as he desired to fly upward or downward. The machine was steered from right to left or left to right by two vertical vanes set at the rear of the machine about a foot apart. They were a little more than six feet long, extending from the upper supporting plane to a few inches below the lower supporting plane. These also were turned in unison by the operator, according to the direction toward which he wished to fly. The most intricate device of their machine, however, was not perfected on their first biplane. This is the one for maintaining a side to side balance, or lateral equilibrium, as the scientists say. In watching the flights of gulls, hawks, eagles, and other soaring birds, the brothers had observed that the creatures, while keeping the main part of their wings rigid, frequently would bend the extreme tips of their wings ever so slightly, which would seem to straighten their bodies in the air. The inventor decided that they needed some such device as nature had given to these birds. The system was called by the scientists the torsional wing system, which means that the tip ends of the wings were flexible and could be warped or bent or curled up or down at will by the operator. Only the rear part of the tips of the wings on the Wright machines could be bent, but this was enough to keep the machine on an even keel when properly manipulated. How the Wright modern machines are operated is fully described on page (99). The whole machine was mounted on a pair of strong light wooden skids like skiis or sled-runners. To start the early Wright biplanes, the machines were placed on a monorail, along which they were towed by a cable. The force for towing them at sufficient speed was obtained by dropping from the top of a derrick built at the rear of the rail a ton of iron which was connected with the cable. The later Wright biplanes were equipped with rubber-tired wheels mounted on the framework, which still retained the skids. Heavy rubber springs were provided to absorb the shock. With the wheels the machine could run over the ground of its own power and thus the cumbersome derrick and monorail were done away with.