[Illustration: PL. 68 AUTOMATIC EQUILIBRIUM DEVICES] [Illustration: PL. 69 FIG. 1 FIG. 2 MECHANISM OF CONTROL] A mechanism which had been devised by the writer for another, but somewhat similar, purpose seemed to be well adapted to this end, and it was accordingly decided to construct a small model of such a size as would be suitable for use on one of the steam-driven models. The plan of control which it was proposed to follow was to have some mechanism which would control the angle of the tail through the action of gravity on a pendulum bob. Since it would require an exceedingly heavy pendulum should the deflections of it be directly utilized to produce corresponding movements of the tail, the most feasible plan seemed to be to have a light pendulum, which, while free to move under the action of gravity, would nevertheless by its movement cause some outside force to produce corresponding and simultaneous movements of the tail. The general scheme of arrangement is shown in Plate 69, Figs. 1 and 2. This device consists essentially of a cylinder (1) in which is mounted a piston with the piston rod (3) passing through the cylinder head and connected to the cord (5) which passes over the pulley (6), fastened to the tube (2), which is slidably mounted on the midrod (7), whence it is carried over the pulley (8) on the guy-post (9). From here it is connected to the spring (10) which is fastened by the [p213] bridle (11) to the upper side of the Pénaud tail (12). The other end of the piston rod (3) passes through the head in the other end of the cylinder, and has connected to it a cord (14) which passes over the pulley (15) fastened to the tube (2), whence it is continued over the pulley (16) and is joined to the spring (17), which is connected by the bridle (18) to the lower side of the tail. Mounted on top of the cylinder (1) is a valve chamber (20) having ports leading to the two ends of the cylinder. Mounted in the valve chamber is a rocking valve surrounded by a bushing having ports in it, and to which is fastened a rod (25) which passes through the said valve and the head of the valve chamber. Fastened to the rod (25) of the bushing is a lever (26), which by means of the link (27) is connected to the piston rod (3). Fastened to the rocking valve is a rod (28) which telescopes over the rod (25) and also passes through the same head of the valve chamber, and carries at its outer end a pendulum (29) on the lower end of which is the bob (30).