io ELEMENTARY SCIENCE 



familiar gas. Our lives are so very closely related to all 

 three of these that it is impossible to say which one of 

 them is most important to us. 



Now let us think of the different ways in which the 

 changes of water from one state to another affect our lives. 

 First, the change from liquid to gas. You know that boil- 

 ing water gives off steam, and you know that even cool 

 water gradually evaporates, which means that it changes 

 into water vapor. There are at least three ways in which 

 this changing of water from liquid to gas affects our lives. 

 Two very important things in our lives are rain and steam- 

 engines. Both of these depend upon the changing of water 

 from a liquid into a gas. Can you explain how ? 



Every boy or girl probably knows that w hen water changes 

 into steam it expands, and expands with much force. Prob- 

 ably you have heard about James Watt, the Scotchman 

 who invented the steam-engine about one hundred and 

 fifty years ago. The story is told that once when he was 

 a boy he stuck a cork in the spout of a teakettle, and the 

 steam blew it out. It blew it out so hard that he began 

 to think about the force behind the blow, and wondered if 

 that force could be used in any useful way. That was the 

 idea that led finally to the invention of the steam-engine. 

 In all steam-engines the fresh steam enters a strong cylinder 

 where it can expand only by pushing a piston, just as the 

 steam in the spout of the kettle pushed the cork. One end 

 of a steel rod is attached to the piston, and the other end to 

 a wheel. As the piston is pushed to and fro by the pres- 

 sure of the steam, the rod turns a wheel, and thus the 

 machinery is operated (see Fig. i). You can see the 

 cylinders and the piston-rods and the driving-wheels on 

 any locomotive. 



