90 HEAT. 



possessing greater elastic force, and the mercury sinks 

 lower, till at 212 the elastic force within, is equi- 

 valent to the pressure of the atmosphere without, and 

 the mercury is pressed down to the external level. 

 Explain the 215. BAROMETER-GUAGE. The princi- 



construction ^tle of the barometer-guage has already 



and use of . & 



the barometer- been explained. A few words will be 

 guage. added here as to its use and construc- 



tion. It is always desirable to know the pressure 

 in a steam boiler, as an evidence of safety, and in 

 order that the fires may be regulated accordingly, 

 and no more fuel be consumed than is necessary. 

 Sometimes the tube containing the quicksilver is of 

 glass, and then the height of the mercury can be seen. 

 In other cases it is made of iron, and the change of 

 level of the quicksilver is indicated by a float. 



216. OTHER STEAM GUAGES. A ther- 



Explain the 



thermometer- mometer may be made to answer, perfectly, 

 guage. ^ Q purpose of a steam guage, as is evi- 



dent from what has been said in paragraph 213. The 

 advantage of such a guage is, that it takes but little 

 room ; its disadvantage, that it is liable to be broken. 

 217. There is still another kind of guage, 



Explain the . 



prfndpie of m which the force of the steam operates 

 another guage. on a m etallic spring, which moves an index 

 more or less, according to the pressure. The spring 

 guage is commonly used in locomotive boilers. 



Explain the ^18. ACTUAL PRESSURE IN DIFFERENT 



di/erence be- ENGINES. The actual pressure of steam, 



twefn high and . 



low pressure used in different forms of the steam en- 

 gine, varies very widely. There are low 



