EFFECTS OP SUDDEN EXPANSION. 



265 



suggested that the outer piece be heated, keeping the in- 

 ner cool, when a force of less than ten pounds quickly 

 separated them. In other cases, where the large iron nuts 

 have been thoughtlessly screwed, w^hile warmed with the 

 hands, on the cold metallic axles of wood-sawing ma- 

 chines in winter, they have contracted so that the force 

 of two or three men has been insufficient to turn them. 



The sudden expansion of bodies by heat sometimes 

 causes accidents. Thick glass vessels, when unequally 

 heated, expand unequally, and break. Heated plates of 

 cast-iron or cast kettles are liable to be fractured by 

 suddenly pouring cold water upon them. The same ef- 

 fect has been usefully applied in splitting the scattered 

 rocks which encumber a farm, and which are too large to 

 remove while entire. Fires are built upon them ; the up- 

 per surface expands while the lower remains cold, and 

 large portions are successively separated in scales, and 

 sometimes the whole rock is severed. The only care 

 needed is to observe attentively 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. 



Fi". 289. 



THE STEAM-ENGINE. 



The Steam-engine owes its power to 

 the enormous expansion of water at the 

 moment it is converted into steam, Avhich 

 is about 1,600 times its bulk when in 

 a liquid state. The principle on which 

 the steam-engine acts may be understood 

 by a simple instrument, represented 

 in fig. 289. A glass tube with a small 

 bulb is furnished with a solid, air- 

 tight piston, capable of w^orking up and 

 12 



