324 FARM MOTORS 



level with the crown sheet, since this is the place where 

 the water must not get low. 



444. Round-bottom types. The principal variation 

 from the original type of this class of boilers is in the de- 

 sign of the rear or furnace end. The common practice is 

 to have the water pass completely around the fire box, 

 including the under side. Such boilers are generally 

 known as the round- or enclosed-bottom type (Fig. 218). 

 As a rule, the draft can enter at front or rear of the fire 

 box. This method of draft frequently aids the fireman 

 in firing up, for when there is but one ash door the direc- 

 tion of the wind may be such as to blow away from the 

 door, retarding the draft. 



445. The open-bottom type (Fig. 219) is so constructed 

 that ash pan and grates can be removed and a complete 

 new fire-box lining put in. The draft can enter at either 

 end of the fire box. There is not as free circulation in 

 this type as in the round-bottom boilers, providing the 

 latter are kept clean. 



When a portable boiler of the locomotive type is setting 

 with the front end low, unless there is an abundance of 

 water, the crown sheet will be exposed and, if not at- 

 tended to at once, will become overheated and collapse. 

 To aid in avoiding this, some manufacturers are making 

 the rear end of the crown sheet (Fig. 220) lower than 

 the front. This mode of construction reduces the size of 

 the rear end of the fire box to a certain degree, but it is 

 done where the space is not essential. Fig. 220 also 

 shows a device which further aids in protecting the crown 

 sheet by displacing the water in the front end of the 

 boiler. 



446. Return-flue boilers of the internally fired type 

 have one main flue, which carries the gases from the 

 fire box through the boiler to the front end. Here they 



