16 EXPERIMENTS IN AERODYNAMICS. 



and when the apparatus is at rest the pencil-point rests in the center of tlie 

 radial springs without pressure upon them, but when any force changes this 

 position of equilibrium it is resisted and measured by the resultant extension of 

 the four radial sj^rings, shown by a definite departure of the pencil from the 

 center in a definite direction. 



The tension of these springs is determined before the apparatus is mounted 

 for trial, by rotating the frame MM' about a longitudinal (imaginary) axis passing 

 through the centers of the wind plane and registry plane. If the pencil end of 

 the arm be w^eighted with (for instance) one pound, it traces out a curve on the 

 paper corresponding to a one-pound tension in every direction. With two pounds 

 another and larger curve is described, and so on till the resultant pi'essure of the 

 four radial springs are then tabulated for every direction and every pressure 

 which the wind of advance may later be expected to exercise. These curves ai'e 

 in practice very nearly circles. 



The distance from the pencil to the gimbals is the same as that from the 

 gimbals to the center of the wind plane, so that the wind pressure, considered as 

 acting at the center of the plane, has the same lever arm as the pressure 

 imposed by the extended springs. It should be particularly noted as a con- 

 sequence of the above-described conditions that, although the wind plane is 

 perfectly free to move in every direction, it is not free to rotate — i. e., it is 

 always during this motion parallel to itself. 



The only other feature of the construction to be noted is the combination of a 

 spring and an electro-magnet connected with the recording pencil. The pencil is 

 held away from the paper by means of the spring until a desired velocity 

 of rotation of the turn-table is attained, when by means of the electro-magnet the 

 pencil is released and allowed to record. 



The method of using the apparatus is as follows : The wind plane is set at 

 an angle of elevation a ; a disk of paper is placed upon the recording board and 

 oriented so that a line diMwn through its center to serve as a reference line is 

 exactly vertical. The wdiirling table is then set in motion, and when a uniform 

 velocity has been attained a current is passed through the electro-magnet and 

 the pencil records its position on the registering sheet. Since gravity is virtually 

 inoperative on the counterpoised plane, the position of this trjice is aifected by 

 wind pressure alone and is experimentally shown to be diametricalh' opposite to 

 its direction, while the radial distance of the trace from the center is evidently a 

 measure of' the pressure on the plane. Thus the instrument shows at the same 

 time the direction and magnitude of the resultant wind pressure on the plane for 

 each inclination of the plane and for different velocities of the whirling table. 

 Since the arms of the apparatus are exposed to the wind of rotation, the outer 

 end, moving with greater velocity than the inner end, will be subject to a slightly 



