1890.] On Wind Pressure upon an Inclined Surface. 253 



nexion with the experiments, but I think that damp air is conducive 

 to a high, and dry air to a low, relative pressure. 



On some days the values will not vary to more than 1 or 2 per 

 cent, throughout ; on others, under apparently precisely similar 

 circumstances, variations of 10 or even 15 per cent, will occur within 

 a few minutes. It is perhaps possible that these changes may be due 

 to variations in the viscosity of the air caused by a change in the dew 

 point, or they may be caused by a circular eddy due to the wind 

 coinciding in position with the path of the pressure plate. The latter 

 supposition seems the more probable, but, if so, it ought to appear 

 sometimes in experiments with velocity instruments, and I have 

 tried several air meters many times, and never detected anything 

 approaching to a variation of 10 per cent. It must be remembered, 

 however, that a change of 5 per cent, in the velocity would produce a 

 change of 10 per cent, in the pressure. 



These variations give an immense amount of trouble, because it is 

 imperative that an experiment should be repeated many times before 

 the mean value is considered correct. 



Actual Pressures. 



The actual value of the pressure for any velocity is obtained thus. 



In the normal position, the pressure for which has previously been 

 denoted by 100, equilibrium was obtained when the moment due to 

 the centrifugal force was 1*33 (ft. and Ibs.). The bar was 27 feet 

 9 inches from the centre of the whirling machine, and the centre of 

 the pressure plate 29 feet 1^ inches ; hence, since the centre of the 

 pressure plate was 1 foot from its axis of rotation, the pressure P is 

 given in Ibs. by the equation 



P_J; 



"27-8x32-2* 



when v is the velocity of the bar in feet per second. 

 This gives 



p _ 1-33 x 27-8V8 

 " 32-2 x (29-1)*' 



when V is the velocity of the plate. 

 Changing to miles per hour, we have 



P = -0029^, 



which gives about 18^ miles per hour as the velocity at which the 

 pressure is 1 Ib. per sq. ft. 



This is, I believe, a lower value than has been previously given. 



