314 THE POWER BEHIND THE ENGINE 



The water thus raised empties into a large tank, built either 

 in the windmill tower or in the garret of the house, and from 

 the tank the water flows through pipes to the different parts 

 of the house. In very windy weather the wheel rotates rapidly, 

 and the tank fills quickly. In order to guard against an over- 

 flow from the tank, a mechanical device is installed which stops 



FIG. 1 86. The windmill pumps water into the tank. 



rotation of the wheel when the tank is nearly full. The tank 

 is usually large enough to hold an amount of water sufficient 

 for several days, and hence a continuous calm of a day or two 

 does not materially affect the house supply. When once built, 

 a windmill practically takes care of itself, except for oiling, and 

 is an efficient and cheap domestic possession. 



Steam as a working power. If a delicate vane is held at 

 an opening from which steam issues, the pressure of the steam 

 will cause rotation of the vane (Fig. 187), and if the vane is 



