CHAP, v.] THE STEAM-ENGINE. 409 



D the outer shell ; E the steam chest ; F the narrowest part of 

 eccentric water space from which the tubes are supplied with water 

 at their lower ends; K the widest part of eccentric water space, 

 through which the upper ends of the tubes deliver the steam pro- 

 duced from the heat absorbed by the tubes and transmitted to 

 the water during its passage through them. The arrangement 

 of tubes is shown at G, Fig. 285, and at H, Fig. 286, and the water 

 spaces shown at F and K. By this arrangement a constant circula- 

 tion is maintained througli the tubes, in the direction shown by the 

 arrows, and by crossing the tubes in alternate layers a constant 

 flow towards and into the lower ends of the tubes is induced, and a 

 constant discharge from the upper ends throughout the other half, 

 thus causing general and uninterrupted currents of water and steam. 



Besides the types just described, there are boilers in which the 

 grate may be removed at pleasure. This arrangement offers ad- 

 vantages of more than one kind, notably that of rapid cleaning and 

 removal of incrustations. There are also circulating boilers, prin- 

 cipally formed of tubes into which water is continually and succes- 

 sively introduced, which vaporizes almost immediately ; and there are 

 toilers worked by heated gas, generally employed in connection with 

 blast furnaces, in which the heated gases escaping from the furnace 

 mouth are utilized. 



Of all these systems of boilers we may notice one which will show 

 us how we may construct steam-generators which are rendered, so 

 to speak, inexplosible, from the fact that the water as soon as introduced 

 is immediately turned into steam. Belleville's circulating boiler, Fig. 

 287, the use of which is spreading considerably in small and moderate- 

 sized manufactories in the populous centres of France is one of these. It 

 is used in many Parisian factories and printing establishments. A series 

 of vertical tubes placed directly over the fire communicates, on one 

 side with a horizontal pipe bringing the supply of water, on the 

 other side with the steam pipe. Each tube is filled with water to 

 the same height, and forms, so to speak, a little boiler half filled 

 with water and half with steam. The flow of water to the tubes is 

 regulated, by means of a special apparatus, by the pressure of the 

 steam itself, so that in proportion as the water vaporizes,- it is re- 

 placed by an equal quantity of water. The level in the tubes of the 

 boiler thus ahvays remains constant. 



