244 H. A. Hazen — Wind Velocity and Pressure. 



therefore, to substitute a heavy weight for the observer and make 

 readings of the scale by standing at the centre and gradually 

 turning so as always to face the scales. Four plates were used, 

 two of them at 16 feet, one at 8, and the fourth at 4 feet from 

 the axis of the apparatus. It will be seen that, if the total pres- 

 sures were proportional to the square of the velocity, that at 16 

 feet would be just 16 times, and at 8 feet 4 times that at 4 feet. 

 It was also possible, by having two plates at 16 feet, to test the 

 effect of different sizes of plates under absolutely the same con- 

 ditions, and this was frequently done, a plate of 16 sq. in. area 

 being compared with one of 576 sq. in. If we take the main 

 bar about \ the length of this just described, and the suspen- 

 sions of the two plates at 16 feet, we shall have the smaller ap- 

 paratus first used. The total pressure upon the plate, by 

 theory, was equal to the weight of the plate multiplied by the 

 tangent of the angle of deviation from the vertical. Careful 

 experiment showed that, within the limits of the angle of swing 

 employed, there were no differences due to the slightly dimin- 

 ished quantity of air meeting the plate with the greater angle. 

 The reason for choosing this method of measurement may be 

 of some interest. After coiling a large number of springs of 

 varying degrees of sensitiveness and calibrating them vertical- 

 ly by the use of weights, after the manner of the Joly gravity 

 apparatus, it was decided to test the accuracy of the calibra- 

 tion on the spring used horizontally by measuring the force 

 necessary to draw out a weight suspended by a long cord. It 

 was found that the calibration did not give the correct value 

 for the force, although all the springs gave almost identical 

 results. The force necessary, or the elongation of the spring, 

 when the angle of the weight with the vertical was small, was 

 too slight, and the contrary was true with larger angles, as com- 

 pared with theory. It was then found necessary to calibrate 

 the springs horizontally by using a pulley, and after that the two 

 curves coincided very satisfactorily. It then appeared that it 

 would be much better to do away with the spring entirely and 

 simply let the plate act like a weight, and then measure the 

 angle of swing. Making the suspending cords of different 

 lengths did not appear to change the result. It was found that 

 the angle of the suspending threads must be quite large, else the 

 plate would take upon itself a wavy motion, twisting back and 

 forth on the line which was its intersection with a vertical 

 plane passing through its center, at right angles to the support- 

 ing bar. 



One peculiarity in the results was the extreme sensitiveness 

 of the plate ; there seemed to be a gradual heaping up of the air 

 in front of the plate as the rotation proceeded, and then a much 

 more rapid falling away causing the plate to drop back. After 



