THE COMPONENT PRESSURE RECORDER. 51 



velocity of the turn-table at the instant for which the reading is made. The 

 occasion for this condition arises in the circumstance that with a varying 

 velocity the inertia of the inner arm of the balance produces a different effect on 

 the instrument from the inertia of the outer arm ; thus with increasing velocities 

 the outer arm tends to go slower than the inner arm, and with decreasing veloci- 

 ties tends to go faster. This differential effect of inertia is taken up by the spring 

 and is combined with the wind pressure until a uniform velocity is attained, and 

 then the wind pressure alone remains to extend the spring. 



Each arm of the balance carries a brass friction wheel, R, which is intended 

 to rest upon a track, P P', thereby limiting the vertical motion of the balance 

 arms. When the wind-plane is vertical, and horizontal wind pressure is being 

 measured, the outer arm carrying the plane rests continuously on the track and 

 the friction wheel affords perfect freedom of horizontal motion of the balance, 

 which fulfills its proper function at the same time that it turns about the vertical 

 axis ; so that when the plane is inclined and is raised by the vertical component 

 of the wind — i. e., when the wind-plane soars — the inner arm is brought down 

 to the stop P and the friction wheel insures free motion of the balance about the 

 vertical axis. An electric wire connects with P, and a second wire carries a 

 current through the knife-edges into the balance, and thence to the friction wheel, 

 where the electric current is completed at the moment of contact between the 

 friction wheel and the stop. After leaving the whirling table the current passes 

 through an electric bell, which serves to inform the experimenter of the fact of 

 soaring (though this is independently recognizable by the motion of the arm), 

 and thence to the observatory chronograph, where the contacts are registered. 

 On this chronograph, then, are registered (1) the second-beats of the mean time 

 standard clock of the observatory ; (2) the contacts, which are made four times 

 in every revolution of the turn-table and show its speed, and (3) the electric 

 current wdiich registers soaring ; the two latter records being clearly distin- 

 guishable. 



The actual method of experiment employed to determine the velocity at 

 which soaring is just attained is as follows: The velocity of the whirling table 

 is increased to the point at wdiich soaring almost begins to take place — that is, 

 when the plane begins to flutter. This velocity is then still further, but very 

 slowly, increased and adjusted until the electric bell rings as nearly as possible 

 half the time. The velocity at wdiich this occurs represents that of soaring. 

 This method is based on the following considerations : If the precise velocity be 

 attained at which the plane would be just sustained in quiet air, not resting on 

 the stop at either end, the actual wind which prevails to a greater or less extent 

 in the open air disturbs this equilibrium and causes the plane to be more than 

 sustained during the half revolution of the turn-table which carries it against 



