AIR AND WATER AS SERVANTS OF MAN 



121 



kyaks of the Eskimos, and the curious basket-like coracles of 

 the Welsh (Fig. 48). 



Probably very early in his primitive life man discovered the 

 value and use of some of the simpler machines such as the lever 

 and the wedge. Much later, he devised the more complex con- 

 trivances to aid him in his tasks. The windmill and the water 

 wheel are among the earliest of them to appear. 



The windmill is a rimless wheel the spokes of which are flat 

 or slightly curved blades set at an acute angle to the plane of the 

 wheel. The principle of operation is simple. When particles 

 of air moving along the surface of the earth as a wind strike 

 these blades, the mill 

 headed into the 

 wind, the force of 

 the blows is resolved 

 into two compo- 

 nents, just as in the 

 case of the kite 

 (p. 80), and one 

 of these component 

 forces turns the mill 

 around. A crank 

 arm attached to the FlG - 48 '~ A coracle 



axle of the wheel which turns with the wheel transmits the power 

 to the pump or other machine to be operated by the mill. 



To make the paper windmill, take a 6-inch square of paper, 

 preferably colored paper. If the paper is not already cut in such 

 form, proceed as follows to cut a 6-inch square out of any rec- 

 tangular sheet of larger size. From any corner of the sheet 

 measure 6 inches along each adjacent side, and mark the points. 

 Fold the corner over and crease the paper along the line connect- 

 ing the marked points. With the scissors, cut the paper close 

 to the folded-over edges. 



Draw lines on the 6-inch square, running from each of two 

 adjacent corners to the diagonally opposite corners. Cut in 



