1890.] On Wind Pressure upon an Inclined Surface. 



that the couple necessary to hold the plate in position alone remains 

 to be determined. 



" Consider the first and second positions of the plate or vane ; sinc<- 

 the wind meets the vanes at the same angles in both cases, the 

 couples would be identical, if the centre of pressure were at the 

 middle of the vane. But it is well known that the centre of pressure 

 is nearer the forward edge, and hence the couples are unequal. 



" If a be the distance from the centre of the plate to its axis of 

 rotation, and if x be the unknown distance of the centre of pressure 

 from the centre of the plate, and if P be the mean pressure estimated 

 over the whole plate, and L t and L 2 the couples corresponding to the 

 two positions, then it is clear that 



"L 1 =P(a-a;); L 2 = 

 " from which we easily get 



" also Pz = (L 2 LJ). 



" Thus, this pair of experiments gives two of the things to be 

 measured. 



" Next consider the 3rd and 4th positions, where the experimental 

 plate is clamped with its plane perpendicular to the arm, and where 

 the inclination to the horizon is complementary to the angle of 

 inclination in the 1st and 2nd positions. 



" Suppose that T is the tangential force on the plate, and L 3 L 

 the couples in the two cases. 



" Then it is clear that 



" L 3 = Ta + Pa and L 4 = Ta-Pa: ; 

 ^'from which we get 



. ....... - 3 4 . .. . 



" If we avail ourselves of the value of P and x, obtained from the 

 1st and 2nd experiments, we have 



"Ta = L 3 - (Lj-LO = L 4 + | (1*-!^). 



" The 3rd and 4th experiments thus afford a redundant equation, 

 and this may be expected to give a check on the consistency of the 

 results with themselves." An expectation unfulfilled. 



For the purposes of comparison, the value of the moment of the 

 1 foot square pressure plate, when exposed normally, with its centre 



