1890.] On Wind Pressure upon an Inclined Surface. 240 



and also with coarse woolly flannel, was tried. So far as I could 

 judge, the sand-paper made no difference in any position, and the 

 flannel made no difference in the normal position. At the angle of 

 maximum moment, the flannel causes a considerable change in the 

 value of the moment. The mean of six experiments made in posi- 

 tions I and II gives a decrease of 21 per cent, in the value of the 

 normal component (not the moment) when the face of the plate is 

 covered with flannel. About the same decrease is caused by tho- 

 roughly wetting the face of the plate, probably because a series of 

 ripples are set up, for just damping it has no appreciable effect. Ex- 

 periments made with the flannel in positions III and IV show that it 

 acts in two ways. It decreases the pressure, and brings the central 

 line of pressure nearer to the centre of the plate ; there is also a de- 

 cided tangential component, but uncertainty about the effect of the 

 eddy in the back positions (I and III) and want of time have pre- 

 vented my making any attempt to determine the numerical value. 



Distribution of the Stream, Lines on the Surface of the Plate. 



In accordance with suggestions made by Professor Darwin and 

 Mr. Buchan on the occasion of their visit to Hersham on March 20th, 

 an attempt has been made to map out the stream lines near the surface 

 of the pressure plate for various angles of incidence. 



For this purpose twenty-five small pins were driven into the face 

 of the board in rows 2 inches apart, and a few short lengths of dark 

 coloured silk were tied to the head of each pin. It was somewhat 

 difficult to see the position taken up by the silk, but the accompany- 

 ing diagrams give a general idea. Each diagram is the result of two 

 drawings made at different times and compared afterwards, and in 

 cases where the two drawings showed much difference a third trial 

 was made. The pace was about 35 miles an hour, that being very 

 near the limit at which the silk could be seen. 



In each diagram the front edge of the plate corresponds to the 

 top, and the left hand to the side nearest to the centre of the 

 whirling machine. The arrows indicate the position taken up by 

 the silk attached to the corresponding pin, and probably show the 

 direction of motion of the air at the point within a few degrees. 

 There is a general tendency of the arrows to incline to the right, 

 which is no doubt due to the centrifugal force. The effect is no 

 doubt indirect, and not due to direct action upon the silk, but to 

 the action upon the comparatively still air, which, being driven to 

 the right, in turn moves the silk. It was evident that in parts 

 : where the symmetry of the diagram was destroyed by a turning to 

 the right, the motion of the air was comparatively gentle, this 

 being seen by the inertness of the threads in those positions. 



VOL. XLVIII. T 



