1890.J On Wind Pressure upon an Inclined Surface. 247 



ments with the natural wind, these values, combined with positions 

 either II or IV, will give the normal component. It will be seen, on 

 reference to the tables of values found for the various positions, that, 

 in whichever way the curve be constructed, a very curious and sudden 

 rise occurs between the angles of 55 and 60. Curve C, fig. 1 0, for the 

 normal component has been constructed by taking the values from 

 positions II and IV, and curve D, fig. 11, by taking the position of the 

 central line of pressure from the natural wind experiments, and then 

 deducing the normal components from the values of the moment in 

 position II. I can form no opinion as to which curve is the more 

 likely to be correct. Some confirmation of the truth of these results 

 is, perhaps, given by the fact that the greatest moment produced by 

 the wind upon a .Robinson cup does not occur when the arm is 

 perpendicular to the wind direction, but when there is a considerable 

 inclination, and the fact that a ship can sail at a good pace when its 

 direction makes with the wind direction an angle considerably less 

 than 90 is worth noting. 



The position of the central line of pressure, as deduced from the 

 experiments made on the whirling machine, is nearer the front edge 

 of the plate than the position given by direct experiment with the 

 natural wind. It will be seen that in both cases the distance of the 

 central line of pressure from the centre of the plate increases more 

 rapidly with the same change in the angle of incidence, as that angle 

 increases in magnitude ; but that the ange at which the acceleration 

 becomes apparent is greater in the natural wind experiments. 



The following results, showing the relation between x and (see 

 preceding table), may be of interest; they are deduced from positions 

 II and IV : 



x = 0-025 foot 6 = 10 



x = 0-050 = 18 



3 = 0-075 = 26 



x = 0-100 = 33 



a; = 0-125 = 39 



x = 0-150 9 = 44 



x = 0-175 = 49 



x = 0-200 = 54 



x = 0-225 9 = 58 



x = 0-250 e = 63 



x = 0-275 = 67 



x = 0-300 = 71 



Hough Surfaces. 



The preceding results refer exclusively to a smooth polished plate. 

 The effect of covering the face with sand-paper of medium coarseness, 



