50 EXPERIMENTS IN AERODYNAMICS. 



The wind-plane is fastened to a brass tube on the outer end of the instrument, 

 and set to any angle of inclination by means of the graduated circle G. This 

 tube is adjustable in position so that the center of the wind-plane, whatev^er be 

 its size, is at a constant distance of 1.25 meters from the center of the balance 

 and of the whole instrument. A similar adjustable tube on the inner arm 

 serves to adjust the balance to equipoise for any position of the outer tube. 

 Beneath the inner arm of the balance a registering arm is rigidly fastened to 

 the vertical axis, and partakes of the horizontal oscillation of the balance, but 

 not of its vertical motion. Near its extremity is attached the horizontal spring 

 already referred to, and at the end it carries a pencil, which registers on a 

 revolving chronograph cylinder below the extension of the spring produced by 

 the horizontal pressure on the wind-plane. 



The length of the record arm from center of balance to spring is 28.5 inches, 

 (72.4 cm.) 



The length of the record arm from center of balance to pencil is 31.5 inches, 

 (80.0 cm.) 



The pencil departures are therefore longer than the true spring extension, 

 and the latter are obtained from the former by multiplying by the factor 



|8^ = 0906. 



To reduce the pull on the spring to what it would be if the spring had the 

 same lever arm as the center of the plane, we must multiply it by the factor 



expressino- the ratio of the lengths of the arms, viz., ^ ~," . , ~ 0.579. 

 ^ ° * I20.O 



Within the limits of attainable precision, we observe the spring calibration 

 to be linear, and the two factors may be multiplied together, giving the single 

 factor 0.524, by which the pressure corresponding to pencil departures, as taken 

 from the calibration curves, must be multiplied in order to get the pressures on 

 the plane. The horizontal springs used in these experiments are those hereafter 

 more fully described in connection with the Rolling Carriage. 



The uniform distance from the center of rotation of the turn-table to the 

 center of wind plane is 9.55 meters. The balance arms are jjrotected fi'om wind 

 by covering the sides of the surrounding frame with cloth and paper and placing 

 over the top an adjustable lid of veneer. An experimental test of the Recorder 

 without wind-plane was first made, to discover the effect of any residual wind 

 pressure on the arms. The instrument was carefully adjusted on the turn-table, 

 and then set in rapid, uniform motion without exhibiting any tension of the 

 horizontal spring. The result indicates that whatever wind pressure still 

 remains is equal on both arms. It is to be noted that a theoretically perfect 

 measurement of horizontal wind pressure by this instrument requires a uniform 



