1890.] On Wind Pressure upon an Inclined Surface. 



243 



these conclusions seeming probable, it appeared advisable to obtain 

 some independent information upon the point, and for this purpose 

 the natural wind was used. 



A light circular disc, of 8 inches diameter, was mounted so that it 

 could turn freely in its own plane, and the lower half was cased in, so 

 that it was completely sheltered. It was then exposed to the wind 

 in various positions, its plane being always vertical, but inclined to 

 the wind direction at different angles. The tangential component of 

 the wind pressure, acting upon one side of the upper half of the disc, 

 would tend to rotate the disc, but it was very seldom that the slightest 

 motion could be obtained, although the friction was so slight that a 

 few grains weight placed on the rim of the disc was sufficient to move 

 it. It seems clear from this that the tangential component is so 

 small that it may be neglected in comparison with the normal, and it 

 has accordingly been considered equal to zero. 



To find the position of the central line of pressure, the arrangement 



FIG. 6. 





