88 



HEAT. 



ter has to grow constantly hotter, in order that steam 

 may form ; and as steam always has the temperature 

 of the water with which it is in contact, the steam 

 grows constantly hotter also. 



212. THE EXACT RELATION OF TEMPE- 



How can the 



exact relations RATURE TO PRESSURE. It is desirable to 



iLr^STpre*- know tne increase of pressure for each ele- 

 sure be deter- vation of temperature. A steam boiler sup- 



mined? ,. . . . _ r 



plied with a barometer guage and a thermo- 

 meter affords the means of ascertaining this rela- 

 tion. Or it may be done by a very small boiler, made for 

 the purpose. The barometer guage is nothing more than 

 a bent tube fitted into the boiler, open to the air at the top, 

 and containing quicksilver in the lower part of the bend. 

 We will suppose all the air to have 

 been expelled from the boiler, the stop- 

 cock through which it made its escape 

 closed, and the whole interior to be 

 filled with steam. As more steam is 

 produced, pressure is increased, and 

 the temperature of both water and steam rise, as before 

 explained. 



What pressure 213. Where the temperature has 

 has steam at rea ched 250, it is found that the pres- 



2oO . at <zti o ? 



at 294 ? How sure of the steam, acting through the water 

 on the quicksilver, is sufficient to force and 

 hold the latter thirty inches higher in one arm of 

 the tube than in the other. But the steam with which 

 the globe was filled when the stop-cock was turned, ex- 

 erted a pressure of fifteen pounds per square inch, just 

 sufficient to balance the pressure of the external air, and 



