CHAP. V.] 



THE STEAM-ENGINE. 



407 



will not be compensated by the advantages of this arrangement. 

 Nevertheless, we employ in England for stationary engines horizontal 

 boilers with one or two interior fires. To further increase the heating 



o 



surface the flues are frequently traversed by tubes crossing each other 

 at right angles, and opening at either end into the interior of the 

 boiler; these also assist greatly in increasing the resistance of the 

 boiler to the pressure of the steam. 



In the greater part of the modifications which the primitive form of 

 boiler has undergone, the chief idea has been to increase as much as 

 possible the heating surface, while economizing the volume and space 

 occupied by the generator. The heaters, the inner and outer flues, 

 the inside fire, have all been invented with the object of utilizing 

 the activity of the fire in such a manner as to let only that portion 

 of the hot gases pass up the chim- 

 ney that is necessary to produce an 

 ascending current, or in other words, 

 a draught. 



Finally, the conception has been 

 gradually arrived at of a tubular 

 boiler, of which the first idea is 

 due to Barlow (1793), but which 

 was not realized till 1829, by 

 Stephenson and Marc Seguin. The 

 system of tubular boilers which 

 was first applied to railways, and 

 has since been adopted in steam- 

 boats with some indispensable modi- 

 fications, is as follows : 



In the principal cylindrical body 

 are fixed numerous tubes parallel 



to each other, which open on one side to the fire and on the other 

 side to the flues or the chimney. The tubes are bathed by the water 

 of the boiler, which fill the intervals between them, and is heated 

 by the gases which traverse the tubes. We shall see further 

 on in what enormous proportion this ingenious arrangement in- 

 creases the heating surface, and in consequence the steam-generating 

 power of the boiler. In locomotives, portable engines, and marine 

 engines, the fire is surrounded on all sides by water, except, of 



FIG. 283. Marine tubular boiler, with 

 return flame. 



