One of the most interesting and ambitious designs was that of Sir Hiram Maxim; and it was one to which he devoted years of labour and large sums of money. He is said, indeed, to have expended £20,000 upon aerial research. After a number of experiments with plane shapes, following the theories of Phillips, he began to build a very large machine, which he set upon a miniature railway as Phillips had done, using the same precaution of a check-rail to prevent it from rising more than a certain distance in the air. His apparatus, when built at Baldwin’s Park, Kent, weighed 8000 lbs.: it was, in fact, the largest machine ever built. The span of its planes was 105 feet, and they offered a total supporting surface of 6000 square feet. [Illustration: FIG. 13. A. Elevating Plane; B.B. Outriggers; C.C. Operating Wires; D.D. Position for ascending; E.E. Position for descending.] The inventor employed the suggestion made by Wenham, and fitted his lifting planes one above the other; while he used a horizontal plane in front of the machine to act as an elevator. This plane could be tilted up and down; and the idea was that, when it was tilted upward as the machine ran forward upon its rails, it would exercise such a lifting influence that the front of the craft itself would be raised, and so cause the main-planes to assume a steeper angle to the air; and the result of the planes being inclined thus more steeply would be to give them a greater lift, and so induce the whole machine to raise itself from the ground. This system is explained in Fig. 13, and is important because such lifting planes, for rising or descending, have now come into general use. Sir Hiram Maxim employed another controlling surface which has become a feature of present-day aircraft, and this was an upright plane, which could be swung from side to side, and by which his craft was to be steered. Such a rudder-plane is illustrated in Fig. 14. By this means, as will be shown later, practically all aeroplanes are steered to-day. The action of the aerial rudder, when it is moved from side to side, is like that of swinging the rudder of a ship; but for the same reason that propellers have to be made large--owing to the small density of the air--so an aeroplane rudder needs to be a comparatively large plane, in proportion to the size of the craft, before it will exercise an adequate turning influence. [Illustration: FIG. 14. A. Vertical steering rudder; B.B. Outriggers; C.C. Operating wires; D.D., E.E. Positions assumed in turning.] To drive his machine Sir Hiram used two specially-built and lightened steam engines, which developed a total of 360 h.p., and yet weighed only 640 lbs.; that is to say, they gave one h.p. of energy for each 1-3/4 lb. of weight. But they were only suitable for purposes of experiment. Sir Hiram himself wrote: