THE COMPONENT PRESSURE RECORDER. 63 



be due to a reversal in the total normal pressure on the planes.* Thus, shape 

 and aspect of plane, while having but slight influence in modifying the pressure 

 when the plane itself is normal to the wind, are most important factors when the 

 plane is inclined. This predominating influence of aspect is, so far as I am 

 aware, now for the first time clearly set forth with quantitative data.f 



BORIZONTAL PRESSURES. 



With every observation of soaring speed, the horizontal pressure on the 

 plane has been measured by means of a horizontal spring. The detailed obser- 

 vations in Tables XIV and XV contain the number of the spring used, the 

 extension of the spring as measured on the trace in inches, the corresponding 

 pull of the spring, measured in grammes, as taken from the calibration curves, 

 and, lastly, the computed pressure on the plane, obtained by multiplying the pull 

 of the spring by the factor 0.524, which reduces the effect of the actually unequal 

 arms of the instrument to what it would have been were the arms equal. For 

 ancles of 90° the instrument affords an additional method of determining the 

 constant of normal pressure, and for all these observations the resulting values 

 of k m and k have been computed. As previously used, the numerical value of k 

 relates to velocities expressed in feet per second and pressure in pounds per 

 square foot, and k,„ relates to velocities expressed in meters per second and 

 pressures expressed in grammes per square centimeter. 



The horizontal pressures on the inclined planes diminish with decreasing 

 angles of elevation, and for angles of 5° and under are less than 100 grammes. 

 Now, for a pressure less than 100 grammes, or even (except in very favorable 

 circumstances) under 200 grammes, the various errors to which the observations 

 are subject become large in comparison with the pressure that is being measured, 

 and the resulting values exhibit wide ranges. In such cases, therefore, the 

 measured pressures are regarded as trustworthy only when many times repeated. 

 On the 30 x 4.8 inch plane, weight 500 grammes, fifteen observations of horizontal 

 pressure have been obtained at soaring speeds. These values have been plotted 

 in Fig. 11, and a smooth curve has been drawn to represent them as a whole. 

 For angles below 10° the curve, however, instead of following the measured 

 pressure, is directed to the origin, so that the results will show a zero horizontal 



* For a further analogy with a corresponding reversal in the position of the center pressure, see Appendix C. 



tOnly after completing these experiments has my attention been called to those of Hutton, who appei 

 have been the first to make experiments in this field, in 1787, and who, it is interesting to see, appreciated the 

 necessity of examining this question of aspect. He tried a plane 8 x 4 inches with both the longedgeand the 

 short edge in the direction of the arms of his whirling machine, but failed to obtain any sensible difference in 

 his resulting horizontal pressure, probably because the friction of his apparatus swallowed up the small differ- 

 ences that exist in the horizontal component of the pressure at small angles. If he had measured the total 

 --.ire or the vertical component, he would probably have discovered a difference in the two cases. I also 

 find that while my experiments have been in progress, Mr. W. H. nine- has likewise been investigating the 

 effect of aspect, at Hersham, England, with results similar to my own. 



