96 EXPERIMENTS IN AERODYNAMICS. 



The results of the calibration were plotted in curves, and these curves have 

 been used for translating all the spring extensions of the experiments into 

 pressures. 



Three square planes were used, 6, 8, and 12 inches on a side, and in every 

 case the center of the plane was placed nine meters from the center of the 

 whirling-table. The air temperature was recorded at the beginning and end 

 of each series of observations. The average wind velocity was obtained from a 

 Casella air meter, which was read each day at the beginning and end of the 

 experiments. It should be noted that these wind velocities are valuable as indi- 

 cating the conditions of experiment, but do not aiford any basis of correction to 

 the observations, since the method adopted in reading the trace eliminates the 

 effect of wind currents, so far as it is possible to do so. In a complete revolution 

 of the turn-table the arm during half of the revolution moves with the wind, and 

 during the other half moves against the wind ; consequently the pressure will 

 be too great during the latter half and too small during the former half of the 

 revolution. Thus, if the velocity at the end of the arm be V, and the wind 

 velocity be v, the wind pressure at one point of the revolution will be propor- 

 tional to {V+vy, and at the opposite point will be proportional to ( V—v)'. The 

 resulting trace, therefore, vibrates on either side of a mean position, and a line 

 drawn through the trace to represent this mean position gives a numerical value 

 that is larger than the pressure due to the velocity V in the ratio of V^ + v^ to VK 

 But, in general, this error in reading the traces is quite negligible, and the average 

 mean position may be taken as reliable within the limits of accuracy imposed 

 on us. The spring extension adopted always refers to this mean position, and no 

 further correction is admissible. A specimen of the records of a series of experi- 

 ments is here given in detail, taken from the note book for October 25, 1888: 



October 25, 1888. 



Barometer, 738 mm. ; mean temperature, 16° C. At 4.53 p. m., air meter, 416,4-15 ; at 5.25 

 p. m., air meter, 419,130. Eight-inch square wind-plane. Spring No. 1. Distance of center of 

 l^lane from axis of rotation, 9 meters. 



First registering sheet. Four records at about 41 revok;tions per minute. Ended at 4.05. 

 Almost a perfect cahii. Velocity too small to get reliable spring extensions. 



Second sheet started at 4.24 p. m. Two records at 10 revolutions per minute. Ended at 

 4.28 p. m. Pencil failed to make satisfactory record. 



