WATER-POWER AND STEAM-POWER 75 



of his devices. It aided him in his first defenses against 

 his enemies. He climbed trees to escape from those of 

 his enemies that could not, by climbing, overcome the law 

 of gravity. Then he built stockades and walls over which 

 his human enemies could not climb. He carried large 

 stones to the tops of walls where they could be conve- 

 niently dropped upon the heads of those who came to at- 

 tack him. Thus he used the force of gravity to crush his 

 enemies. 



But nowadays falling water is much more important to 

 man than falling stones. In ancient days slaves did the 

 work of carrying up the stones. What does the work of 

 carrying up the water? What is the force that takes the 

 water from the seas and lakes and carries it up to the 

 hills, whence it runs back to the seas again, and, in running, 

 turns the wheels of ten thousand mills? Evidently we 

 must think again of that great water cycle of nature de- 

 scribed in Chapter III. And we must ask ourselves 

 what is the force behind evaporation ? We noted in 

 Chapter I that increase of heat hastens evaporation. 

 Now can it be that it is the heat of the sun that 

 does this great work of lifting water up? Is it the sun 

 that we have to thank for getting the water in such po- 

 sition that its fall does work for us ? Does the sun then 

 serve us somewhat as the slaves of old served their masters 

 when they carried stones up to high places ? We shall see. 



Steam-power is like water-power in that water (in the 

 form of gas) is used. It is used to push against something. 

 In the case of steam-engines this pushing or pressure is 

 the pressure of steam in a cylinder against a piston (see Fig. 

 i). The compressed steam forces the piston through the 

 cylinder, then this steam is released. Other steam imme- 



