174 



HEAT. 



the water trickles back into the boiler, so that there is no diminution of 

 its contents. The pressure to which the water in the boiler is subjected 



FlG. 97. Water- Vapour Pressure and Ice-Vapour Pressure. They differ 

 by 0-04 mm. at - 1. 



can be changed either by withdrawing air from, or forcing air into, the 

 reservoir G by the pipe F. Four iron tubes closed below were inserted 



FlG. 98. Dynamical, or Boiling Method of Determining Vapour-Pressnre 

 of Water above 50. 



through the top of the boiler, two reaching nearly to the bottom of the 

 water, and two shorter tubes being surrounded by the steam. These 



