332 Of the Use of Steam 



of the atmosphere, but also the additional pressure of 

 the superincumbent liquid in the vessel ; the steam- 

 boiler must therefore be made strong enough to con- 

 fine the steam, when its elasticity is so much increased, 

 by means of additional heat, as to enable it to overcome 

 that resistance. This increase of the elastic force of the 

 steam need not, however, in any case, exceed a pressure 

 of five or six pounds upon a square inch of the boiler, 

 or one third part y or one half, of an atmosphere. 



It is not necessary for me to observe here, that in 

 this and also in all other cases where steam is used as a 

 vehicle for conveying heat from one place to another, it 

 is indispensably necessary to provide safety-valves of 

 two kinds, the one for letting a part of the steam 

 escape, when, on the fire being suddenly increased, the 

 steam becomes so strong as to expose the boiler to the 

 danger of being burst by it; * the other for admitting 

 air into the boiler, when, in consequence of the diminu- 

 tion of the heat, the steam in the boiler is condensed, 

 and a vacuum is formed in it ; and when, without this 

 valve, there would be danger, either of the sides of the 

 boiler being crushed, and forced inwards by the pressure 

 of the atmosphere from without, or of the liquid in 

 the containing vessels being forced upwards into the 

 horizontal steam-conductors, and from thence into the 

 steam-boiler. The last-mentioned accident, however, 

 cannot happen, unless the cocks in some of the steam- 

 tubes are left open. The two valves effectually pre- 

 vent all accidents. 



* The steam which escapes out of the boiler through the safety-valve may very 

 easily be made to pass into the reservoir of water which feeds the boiler, and be con- 

 densed there ; which will warm that water, and by that means save a quantity of heat 

 which otherwise would escape into the atmosphere and be lost. 



