3^6 *ARM MOTORS 



are divided and enter several smaller flues, then return 

 to the rear end and pass up the stack. This, without 

 doubt, is a very economical type. 



By referring to Fig. 221, which is an end view of a 

 return-flue boiler, it will be noticed that the smaller tubes 

 are above the main flue. By this arrangement the smaller 

 and cooler parts will become exposed first, thus giving 



FIG. 219 OPEN-BOTTOM FIRE-BOX BOILER 



the engineer a chance to save the boiler from collapse or 

 explosion. 



447. Wood and cob burners. Most boilers upon the 

 market have interchangeable grates so that by placing a 

 grate with smaller openings in place of the coarser one 

 for coal, wood and cobs may be burned. 



Since the most economical firing can be accomplished 

 by refraining from poking the fire on top, a great many 

 factories are making a rocker grate (Fig. 222), which is 



