XLII ADMINISTKATIVE REPORT [eth. ann. 20 



tool used only by human power; the thrasher is a raachme in 

 which horse power or steam power is employed. 



In the miiltiplicatiou of processes, which we have already 

 illustrated somewhat, many machines are employed in the 

 manufacture of a single class of products. Often these machines 

 are housed for their protection and for the protection of the 

 laborers who are operating- them. Such a group of machines 

 with their houses is called a mill or a factory. In the mill many 

 machines may be used, and many tools, all designed for the 

 common pur])Ose of producing a class of objects. 



It now remains for us to set forth the classes of powers 

 which are used by men to promote their welftu'e. These are 

 muscular power, wind power, water power, heat power, and 

 electric power. 



Mnscidar poiver. This power is the primordial force used 

 by mankind. It was used first as human power, but in the 

 second stage of human culture animals were domesticated and 

 used as beasts of burden. Especially is one animal used for 

 this purpose, namely, the horse, and the power of a horse for 

 a definite period of time, established conventionally, has come 

 to be used as the standard of measurement for powers. Ani- 

 mals are used not only for carrying and hauling burdens, but 

 they are used also for impelling machinery. 



Wind power. Wind power is used to propel machinery and 

 especially in the navigation of water to propel vessels, and the 

 macliinery devised for the latter purpose consists of masts and 

 sails. In the early history of civilization the propulsion of 

 vessels and the running of mills were relatively' much more 

 common than at present, and yet this power is widely used. 

 Since air lias been licjuified it seems likely that this substance 

 is to play a still more important role in mechanics, and that 

 air is to become a commodity. 



Water poiver. Water power is used chiefly for the I'unning 

 of mills. The tides as they rise and fall are utilized in their 

 onward rush to impel mills by the construction of the neces- 

 sary machinery, and the fall of water in running streams is 

 utilized for the same purpose. Water is used also as steam to 

 connect heat i)ower with machinery. 



