OR, PLAIN TEACHING. 



97 



STEAM— THE STEAM ENGINE. 



368. 



True steam is invisible, like 

 nir. That which we see escaping 

 from kettles, urns, and other ves- 

 sels is vapour, caused by steam 

 losing its heat, and returning to 

 the state of water. 



A drop, or globule of water, 1, 

 heated to the boiling point, and 

 i changed into steam, would 

 occupy as much space as 

 \ seventeen hundred of those 

 lilfe^/ drops. Ten times the 

 369. space, 2, occupied by the 



2 



370. 



diup, 1, shows a considerable in- 

 crease, or expansion. But the one 



drop converted into steam, would 

 occupy a hundred and seventy 

 times as much space as is represen- 

 ted by the ten circles, 2. There- 

 fore, the expansion is enormous. 



In order to obtain this expan- 

 sion, steam exerts considerable 

 force. If steam 

 were not allowed 

 to escape from 

 an ordinary ves- < \\. 

 sel, 3, containing 

 boiling water, it 

 would exert ^ ^ 

 such a force, 

 that it would : f^^>^S 

 burst the vessel , '^^^^^^\ 

 throw fragments 

 of metal with MM^m^ 

 violence to a 371. 

 great distance, and suddenly fill 

 the air with volumes of vapour. 



