182 



THE POWER BEHIND THE ENGINE 



the tank the water flows through pipes to the different 

 parts of the house. On very windy days the wheel rotates 



FIG. 125. The windmill pumps water into the tank. 



rapidly, and the tank fills quickly ; in order to guard against 

 an overflow from the tank a mechanical device is installed 

 which stops rotation of the wheel when the tank is nearly 

 full. The supply tank is usually large enough to hold a supply 

 of water sufficient for several days, and hence a continuous 

 calm of a day or two does not materially affect the house flow. 

 When once built, a windmill practically takes care of itself, ex- 

 cept for oiling, and is an efficient and cheap domestic possession. 

 178. 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 (Eig. 126), and if 

 the vane is connected with a machine, work can be obtained 

 from the steam. 



