212 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



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 at- 

 tached. 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 con- 

 nections, 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 some- 

 thing 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 Vallom- 

 brosa, 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. 



A mechanism which had been devised by the writer for another, but some- 

 what similar, purpose seemed to be well adapted to this end, and it was accord- 

 ingly 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 pro- 

 posed 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 re- 

 quire 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 he to have a lighi 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 gen- 

 eral scheme of arrangement is shown in Plate (>'.K Pigs. 1 and L>. This device 

 consists essentially of a cylinder (1) in which is mounted a piston with the 

 piston rod (•'!) 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 



