THE COMPONEiNT PRESSURE RECORDER. 49 



to its vertical oscillations about the knife-edge bearing, it is a true balance, whose 

 arms, each one meter long, are in delicate equilibrium, and I will call this part 

 of the instrument distinctively " the balance." 



If an actual working aerodrome model with its motor be not used upon 

 the outer arm (outer, that is, as reckoned from the center of tlie turn-table), a 

 plane of given weight (the "wind-plane") is clamped there, so as to make any 

 desired angle of inclination with the horizon. The horizontal oscillation about 

 the vertical axis provides for the measurement of the horizontal component of 

 pressure on this plane ; the vertical oscillation on the knife-edge provides for 

 measuring the vertical component. The horizontal pressure is measui-ed by the 

 extension of a spring fastened to an arm moving around the axis with the 

 horizontal oscillation of the balance, and to the surrounding fixed frame. The 

 vertical component of pressure is measured«only when it is equal to the weight 

 of the i^lane — i. e., by the fact that the plane is actually just lifted by the wind 

 of rotation, or, in the technical term previously used, when it soars. The requisite 

 registration of this fact is automatically accomplished by making an electric 

 contact. As the wind-plane is raised, the inner end of the balance descends, until 

 it strikes a stop through which electric connection is established, and the 

 " making " of the current is registered on the stationary chronograph, which 

 at the same time records the speed of the whirling table four times in each 

 revolution, and thus the horizontal velocity which produces a vertical pressure 

 sufficient to lift or sustain the wind-plane is determined. 



The detailed manner in which these objects are attained by the apparatus 

 is described later in the text, and is shown by the drawings of plate VII. The 

 letters S designate the iron supports by means of which the frame of the recorder 

 rests upon the arm of the whirling table in such a manner that the instrument 

 is half above and half below it. The knife-edge and the wind-plane are brought 

 thereby into the plane of rotation, and equal surfaces above and below the 

 supporting arm of the whirling table are exposed to the wind pressure. 



The details of the knife-edge bearings are shown on the jilate in enlarged 

 scale. It is evident that when the balance resting on its knife-edge is in motion 

 on the whirling table, there will be an outward thrust on the instrument tending 

 to throw the knife-edge off from its bearing. In order to take up this thrust, 

 and yet in no way impair the action of that portion of the instrument which 

 acts the ])art of a balance, a pair of cylindrical pivots exactly concentric with the 

 prolongation of the knife-edge are made to extend out beyond the knife-blade 

 and rest in a suitable bearing. The pivots thus arranged take up the outward 

 thrust arising from centrifugal force, while the freedom of motion of the balance 

 on the knife-edge is not at all impaired. 



7 



