Adopting the scheme of superposed surfaces then recently devised by Wenham, Mr. Stringfellow in 1868 constructed the interesting steam-driven model shown in Plate XIII. This consists essentially of three superposed planes, rigidly connected by rods and diagonal wires, propelled by a pair of screws actuated by a high pressure steam engine, and guided by a tail. The three planes aggregated 21 feet in length and 28 square feet in surface; totaling, with the tail, 36 square feet. The engine was rated at one third of one horse power. Its weight is not known, but may be roughly surmised from the fact that a separate engine exhibited simultaneously by Stringfellow weighed thirteen pounds per horse power. The model was entered for competition in the London Aëronautical Exhibition of 1868. In actual operation, however, it seems not to have excelled the monoplane of 1846; but still it is of much interest as being the prototype of the multiple-wing aëroplane now in common use. It seems to have been the first aëroplane having two or more sustaining surfaces joined by rods and stayed by diagonal cords after the manner of a Pratt truss. This historic little model was purchased by Professor Langley for the Smithsonian Institution, and is now to be seen suspended from the ceiling of the National Museum, beside Langley’s own models and Lilienthal’s epoch-making glider. [Illustration: FIG. 32.—PENAUD’S AËROPLANE TOY, 1871.] In 1871 M. A. Penaud produced the interesting toy aëroplane shown in Fig. 32. The model is propelled horizontally forward by a single screw, actuated by twisted rubber, and is fastened, as shown, to the middle of a long stick or backbone. The center of mass of the machine is well to the front, tending to plunge the model earthward like a heavy-headed arrow; but this down-diving is promptly checked by the tiny rudder which is so inclined as to counteract the diving proclivity. That is to say the rudder dips so as to receive the aërial impact on its upper surface; which impact increases with the speed of flight and causes the bow to rise, until the weight before the wings just balances the impact on the rudder at the rear. The equilibrium is thus automatic, on the principle expounded by Sir George Cayley sixty years earlier. This quaint little bird when liberated in the Garden of the Tuileries flew a distance of 131 feet in eleven seconds, much to the delight of some members of the French Society for Aërial Navigation. It may be added that Penaud, who was a most promising and clever aëronautical inventor, contemplated a twin-screw monoplane large enough to carry two men, but died in his early manhood, before the project could be realized. [Illustration: FIG. 33.—TATIN’S AËROPLANE MODEL, 1879.]