SPINNING MOVEMENT. 



draught of air upwards or downwards, acting 

 upon the obliquely set vanes in the same way 

 as on the sail of a windmill, or as the sail of 

 the ice-boat described in the last section, 

 should cause these vanes to revolve. In an 

 up-draught, the air, striking upwards against 

 the lower surface of the oblique vane S T, 

 will drive it from right to left, while the vanes 

 at the back are similarly driven from left to 

 right. A down-draught, on the other hand, 

 striking T S on the upper surface, will drive 

 it to the right, while the back of the disc is 

 driven to the left. 



As soon as you mount the disc on the pin- 

 point, and hold it up steadily, you find that 

 it begins at once to revolve, the front turning 

 to the left. Apparently just where you stand- 

 there is an up-draught, which was perhaps 

 too slight for your previous observation. 



Go to another part of the room, where you 

 expect to find no draught at all, and hold the 

 disc steadily in position there. It begins again 

 to revolve, and in the same direction as before. 

 Apparently there is an up-draught there also. 

 If you try yet other parts of the room you get 

 the same result. 



If a window or ventilator be open to a strong 

 wind, and swung at such an angle as to direct 

 the blast downwards, you then get a powerful 

 local down-draught, which at once sets the 

 disc spinning in the opposite direction. 



c 17 



