CHAP, v.] THE STEAM-ENGINE. 405 



they do not offer the same guarantee for exactness that the others do, 

 because the pieces submitted to the pressure of the steam may alter 

 by use. Their action depends upon the metallic rods indicating by 

 the greater or less curvature impressed upon them by the elastic force 

 of the steam the value of this force, but it is necessary from time to 

 time to submit them to verification by a comparison with more exact 

 manometers. The disadvantage of the latter 

 arises chiefly from the material of which they are 

 composed, namely, glass, which gets dirty and 

 loses its transparency, but through which one must 

 read the mercury ; their fragility forms another 

 objection. The mercury, too, in the compressed 

 air manometer becomes oxidised, which diminishes 

 the volume of the air ; so that the indicated pres- 

 sures are greater than the true ones; they are 

 also obviously inapplicable to locomotive engines. 

 Such then, in its essential parts, is the steam- 

 generating apparatus known in practice under the 

 name of boiler. The boiler varies much, as 

 already stated, in its dimensions and shape, according to the kind 

 of engine to which it furnishes the motive force. We shall notice 

 successively the most common and most original arrangements 

 of boilers employed for stationary engines, marine engines, and 

 portable engines and locomotives. 



V. THE PRINCIPAL TYPES OF STEAM-BOILERS. 



In the boiler with heaters we have just described, the boiler is 

 over the fire it is a generator with an exterior fire. There are also 

 generators with interior fires, and upon this single difference we may 

 form two types of boilers, which each divide up into numerous varie- 

 ties. Lastly, we may distinguish a third type, that in which the fire 

 properly so called is exterior, but the flues or conduits for the gases 

 of combustion are lodged in the interior of the chamber containing 

 the water. 



The first form adopted in Watt's engines was the so-called waggon 



