1890.] On Wind Pressure upon an Inclined Surface. 257 



in the intermediate positions is necessarily greater numerically than 

 the normal component. 



Curve H, 6g. 13, gives the normal component of the pressure upon 

 the same strip of wood with the longer axis inclined to the wind. In 

 this case the wood was mounted 1 foot from the lever (EF, fig. 1) 

 and parallel to it. The curve is drawn from the values of the 

 moments in positions I and II, want of time having prevented my 

 making observations in positions III and IV. The curve shows the 

 normal component, and not the moment, and the distances of the 

 centre of pressure from the centre of the strip are given in the 

 following table, x and 6 having the same meanings as before : 



x = 0-10 foot 6 10 



a? = 0-24 6 = 20 



x = 0-36 = 30 



x = 0-40 40 



x = 0-37 6 50 



x = 0-39 e = 60 



It is probable that the flattening of the curve about the angle of 

 50, and the corresponding departure from symmetry in the relation 

 between x and for that angle, is accidental. The curve is obviously 

 wrong at 90, where the normal component should be zero, but I have 

 sought in vain for any explanation of this, and can only put it down 

 to the eddy from the frame, which has caused so much trouble in 

 other ways. It is not due to any want of balance, so far as gravity 

 is concerned. Several trials have been made during the course of 

 the experiments to see whether the sliding bar would come to the 

 zero mark when the plate and balance weight were removed from the 

 apparatus, and it has always been found to do so with very fair 

 accuracy. Care has been taken to see that the moving parts were 

 properly balanced for weight, and although the pressure upon the 

 plate, and consequent deflexion, has no doubt slightly altered the 

 balance while the apparatus was moving, the error so caused is very 

 trifling, at least so far as the foot square plate is concerned. 



In conclusion, I must say that I am sorry not to have been able to 

 clear up the different inconsistencies better, and not to have been 

 able to draw the curves for the normal component with a greater 

 certainty of their being accurate. 



