Two propellers are used, run by one motor. They are built of wood, are of the two-bladed type, and are of comparatively large diameter--8 feet. They revolve in opposite directions at a speed of 450 revolutions per minute, being geared down by chain drive from the engine speed of 1,500 revolutions per minute. The large elevator planes in front have been a distinctive feature of the Wright machine. They have a combined area of 80 square feet, adding that much more lifting surface to the planes in ascending, for then the under side of their surfaces is exposed to the wind. If the same surfaces were in the rear of the main planes their top sides would have to be turned to the wind when ascending, and a depressing instead of a lifting effect would result. To the rear of the main planes is a rudder composed of two parallel vertical surfaces for steering to right or left. The feature essential to the Wright biplane, upon which the letters patent were granted, is the flexible construction of the tips of the main planes, in virtue of which they may be warped up or down to restore disturbed equilibrium, or when a turn is to be made. This warping of the planes changes the angle of incidence for the part of the plane which is bent. (The angle of incidence is that which the plane makes with the line in which it is moving. The bending downward of the rear edge would enlarge the angle of incidence, in that way increasing the compression of the air beneath, and lifting that end of the plane.) The wing-warping controls are actuated by the lever at the right hand of the pilot, which also turns the rudder at the rear--that which steers the machine to right or to left. The lever at the left hand of the pilot moves the elevating planes at the front of the machine. [Illustration: Sketch showing relative positions of planes and of the operator in the Wright machine: _A_, _A_, the main planes; _B_, _B_, the elevator planes. The motor is placed beside the operator.] The motor has 4 cylinders, and develops 25 to 30 horse-power, giving the machine a speed of 39 miles per hour. A newer model of the Wright machine is built without the large elevating planes in front, a single elevating plane being placed just back of the rear rudder. This arrangement cuts out the former lifting effect described above, and substitutes the depressing effect due to exposing the top of a surface to the wind. [Illustration: _Courtesy of N. Y. Times._ The new model Wright biplane--without forward elevator.] The smallest of the Wright machines, popularly called the “Baby Wright,” is built upon this plan, and has proven to be the fastest of all the Wright series. THE VOISIN BIPLANE.