In an early and simple form, the aerial propeller was as shown in Fig. 7. Here are two curved blades, so shaped that, when the propeller is made to revolve quickly, these blades will act powerfully upon the air. What the propeller does is to screw itself forward through the air, as one might revolve a corkscrew and drive it into a cork, or force a gimlet into a piece of wood. Each time you twist the gimlet for instance, as you drive it inwards, it forces itself a certain distance through the wood; and in a like manner the air-propeller, each time it revolves, tends to bore its way through the air (Fig. 8) and so push, or draw with it, the flying machine to which it is attached. But with air, seeing that its density is small, it is necessary to use a large screw, and to turn it fast, before power can be obtained. [Illustration: FIG. 8.] In 1845, Stringfellow, who was now working alone--Henson having abandoned the tests and gone abroad--met with a definite success. He obtained actual flights with a steam-driven model in the form of a monoplane, weighing 8-1/2 lbs. These tests attracted attention among scientists, but they led to nothing else--that is to say, no full-sized machine was the result. But Stringfellow’s model interested many people in the problems of flight. It showed, indeed, although in miniature, that a flying machine could be built and driven through the air; and so this patient experimenter did not labour in vain. Following Stringfellow, upon the list of those who forged links in the aerial conquest, came Francis Herbert Wenham. His interest in flying, as with many other men, was aroused by watching the birds. Wenham, an engineer by profession, made a voyage up the Nile; and his study of the movement of birds, as they flew near his yacht, caused him to take up aviation in earnest, and carry out experiments for the Aeronautical Society. Wenham was interested largely in the lifting power of planes, and sought efficient shapes. He recommended the building of arched surfaces, so arranged that they had considerable span, but were narrow from front to back; and he suggested also that they should, when fitted to a machine, be placed one above another. Thus Wenham was the inventor of the biplane, as we know that craft to-day. In explaining this point he wrote: “Having remarked how thin a stratum of air is displaced between the wings of a bird in rapid flight, it follows that, in order to obtain the necessary length of plane for supporting heavy weights, the surfaces may be superposed, or placed in parallel rows with an interval between them” (Fig. 9). [Illustration: FIG. 9.--Superposed Lifting Planes.]