318 FARM MOTORS 



verted into steam. The water of a low-pressure boiler, 

 i.e., one which carries a pressure of only about 5 pounds 

 gauge, is heated to only about 228 when steam is given 

 off, while in a high-pressure boiler which carries about 

 200 pounds gauge pressure it has to be heated to about 



385. 



The first boilers were simply large cylindrical shells. 

 They did the work required of them, but were very in- 



FIG. 212 VERTICAL BOILER 



FIG. 213 VERTICAL BOILER 

 WITH SUBMERGED FLUES 



efficient. The next was merely a shell with one tube 01 

 flue, as it is often called. Multitubular, return tubular, 

 internally fired, water-tube, sectional boilers, etc., have 

 come in in succession until we have the present-day 

 types. 



439. Classification. Steam boilers may be classified ac- 

 cording to their form and use. Thus we have locomotive, 



