In the earlier period of the work and before the correct application of the Pénaud tail to the model aerodromes had been found, Mr. Langley had planned a large number of different forms of automatic control for preserving the equilibrium of the machines. The more frequently recurring of these were devices for changing the angle of the wings or tail, and others for shifting the wings or tail bodily so as to shift the position of the center of pressure with respect to the center of gravity, the motive power for operating the devices being in some cases that derived from a gyroscope or a pendulum, and in others small electric motor apparatus controlled by a pendulum or a gyroscope. Most of these, however, never reached the stage of development where they were actually tried on the machines in flight, as the tests of some of them in the shop showed that they were unreliable, while others were abandoned either when partly built or when only the drawings for them had been made. Among the better-preserved models of devices for this purpose which were in existence when the writer became associated with the work are those shown in Plate 68, [p212] where the piece at the top is a pendulum (inverted or direct) which controls the movement of the horizontal tail by means of the cords and apparatus shown, actuating these through the small electro magnets and apparatus attached. Just below the rod, which represents a piece of the midrod, are three parts, the first of which is a group of six little batteries clustered in a circle, while next to it is a system of needles hung in gymbals, with electro-steering apparatus in cups which itself turns on a graduated base, these electric connections, together with the battery, controlling the vertical rudder. On the right of this is another piece of apparatus for actuating windlass cylinders which turn one way or the other as the contact is made by one side or the other of the pendulum or the needle. At the bottom, on the two rods, is a tail-piece which automatically throws the center of pressure forward or backward according as the aerodrome departs one way or the other from the horizontal. In spite of the fact that all the early attempts of Mr. Langley to devise such a mechanical control had been very unsatisfactory, the idea that something of this kind was necessary had never really been abandoned by him. Here was to be seen one of his chief characteristics, which was never to abandon any idea that seemed valuable until it was brought to a successful issue or some very strong proof was developed that the idea was impracticable. While on a trip abroad during the summer of 1899, and especially while resting at Vallombrosa, Italy, Mr. Langley’s mind again turned to this problem, and he wrote a number of very interesting letters emphasizing the importance of devising such a mechanism which should be controlled by gravity. When he returned to the Institution in the fall he insisted upon the same idea.