20 



Trans. Acad. Sci. of St. Louis. 



may then determine K. The total effective force against the 

 plane, we have found to be 118.7 lbs. Since the area was 

 12 sq. ft. the average pressure is 9.89 lbs. per sq. ft. Hence 

 K = 9.89 -4- 7.17 = 1.37. 



The pressures in Fig. 3 correspond to a balance pull of 

 100 lbs. on an arm of 3 ft. This is equivalent to a force of 

 (3 h- 2.5) 100 = 120 lbs. at the center of pressure. The 

 pull per square foot is therefore 10 lbs. This gives a value 

 for K of 10 -5- 7.17 = 1.39. This value is only in part de- 

 termined from data independent of that involved in the other 

 determination. 



If instead of taking the pressure at the middle of the pres- 

 sure board we take the average pressure over the front face, 

 we have by the spring balance determination of pull, K — 10 

 -f- 5.58 = 1.79, which agrees very well with some of the 

 higher values of K determined by others. It is however evi- 

 dent that this value of K is theoretically a wholly different 

 quantity from that which we seek to determine. 



It seems entirely possible that the pressure at the center of 

 the board is less than that which corresponds to the average 

 (velocity) 2 with which the wind sweeps past the board, as 



