An additional point is needed to explain how motors were lightened for aviation work. The fly-wheel has been described as vital to the engine, and so it is, unless a number of cylinders are used. But if a maker builds a motor with, say, eight cylinders, the driving impulses are so frequent that there is no danger of the engine ceasing to revolve between explosions, even if no fly-wheel is fitted. It can be so arranged, indeed, by the timing of the explosions, that there is a smooth, even thrust upon the crank-shaft, and by omitting the fly-wheel there is a perceptible lessening in the weight of the motor. When the Wrights had built an engine, there was still the question how they should make it drive their aeroplane. They inclined naturally to the idea of an aerial propeller such as that illustrated in Fig. 7. Two courses lay open to them; they could fit one propeller running at high speed and coupled directly to the motor, or they could use two propellers, revolving at slower speed and geared in some way to the engine. They decided upon the latter course, placing two propellers behind the main planes of their machine and driving them from the engine by means of light chains, these running in guiding tubes. This system of propulsion is shown in Fig. 33. [Illustration: FIG. 33.--Wright Motor and Propellers. A. Motor; B. Gear-wheels upon motor crank-shaft; C.C. Tubes carrying driving chains; D.D. Sprocket-wheels over which chains pass; E.E. Propellers.] The propellers revolved in opposite directions, and in order to gain this effect one chain, as will be seen from the sketch, had to be crossed in its tube. The Wrights preferred the use of two propellers, even though this necessitated gearing such as might have been avoided had a single propeller been coupled directly to the motor. They considered the thrust upon the aeroplane would be smoother with two screws. By using a couple of large propellers also, and running them rather slowly, they reckoned to obtain more efficiency than with a single high-speed screw. In illustration of this it may be mentioned that, whereas the single propellers upon some of the French machines made 1000 or 1500 revolutions a minute, the Wrights’ two screws made only 450 a minute. That their system was extremely efficient they were able to prove beyond question. Many critics condemned as clumsy the use of gearing chains, and pointed out the danger that existed should a chain break--causing one propeller to stop, while the other, by continuing its thrust, tended to throw the machine out of equilibrium. Such a system, indeed, never came into general use, but in fairness to the Wrights it should be said that very few accidents were ever attributed to the breaking of a chain. [Illustration: FIG. 34.--The Wright Biplane. A.A.--Main-planes; B. Double front elevator; C. Rudder (two narrow vertical planes); D. Motor; E. Propellers; F. Pilot’s lever; G. Skids upon which machine landed.]