THE STEAM-ENGINE. 



241 



The sudden expansion of todies by heat sometimes 

 causes accidents. Thick glass vessels, when unequally 

 heated, expand unequally, and break. Heated plates 

 of cast iron or cast kettles are very Hable to be frac- 

 tured by suddenly pouring cold water upon them. The 

 same effect has been usefully appUed in sphtting the 

 scattered rocks which encumber a farm, and which 

 are too large to remove while entire. Fires are built 

 upon them ; the upper surface expands, while the low- 

 er remains cold, and large portions are successively 

 separated in scales, and sometimes the whole rock is 

 severed. The only care needed is to observe atten- 

 tively and remove with an iron bar any parts which 

 may have become loosened by the heat, and which 

 would prevent the heat from passing to other portions. 

 One man will thus attend to a large number of fires, 

 and will split in pieces ten times as many rocks in a 

 day as by drilling and blasting. 



Fig. 202. 



THE STEAM-ENGINE. 



The Steam-engine owes its power to 

 the enormous expansion of water at the 

 moment it is converted into steam, which 

 is about 1600 times its bulk when in the 

 form of water. The principle on which 

 the steam-engine acts may be understood 

 by a very simple instrument represented 

 in Fig. 202. A glass tube with a small 

 bulb is furnished with a solid air-tight 

 piston, capable of working up and down. 

 The water in the bulb, a, is heated with 

 a spirit-lamp or sand-bath ; the rising 

 L 



