THE STEAM TRACTOR 47 



made. The straw-burning attachment consists of a rather 

 long feeding chute with a trap door, a dead plate upon which 

 the straw drops, short grates, and a brick arch which deflects 

 the draft toward the incoming straw. The draft is consider- 

 ably reduced, all the straw is consumed in the fire box, and the 

 gases are fully heated before entering the flues. 



The boiler jacket, which may be composed of wood, asbestos, 

 air spaces, or other non-conducting media, encased in a gal- 

 vanized steel covering, renders the boiler from 6 to 10 per 

 cent, more efficient. Considering the exposed nature of the 

 work, every plowing engine should be thus protected. 



The crown sheet i.e., the top of the fire box is arched 

 in the best plowing engines in order to resist the pressure of 

 the water and steam above it. The bottom of the fire box is 

 either open or closed. If closed, the fire box is usually sur- 

 rounded entirely by water. This gives additional heating 

 surface and better circulation, provided the space at the 

 bottom is kept free of sediment. In the open-bottom fire 

 box the water legs, which extend down on all sides, are closed 

 at the bottom. This allows the grates to be set lower, and 

 the combustion chamber thus enlarged. It also allows easy 

 removal of the ash pan and grates, for cleaning or relining the 

 fire box. The draft may be admitted from either front or 

 rear without reducing the heating surface. The fire box walls 

 proper are held in place by tight-fitting threaded stay bolts, 

 screwed through the plates of the boiler shell and the fire 

 box. A set of rocker grates is usually installed, so that the 

 fire may be kept clean without poking from the top, which 

 invariably causes a loss of heat in unburned carbon. 



Owing to the small grate area which can be profitably pro- 

 vided in a traction engine, it is necessary to have some means 

 of increasing the draft. Engines are usually provided with a 

 blower through which live steam can be passed into the smoke 

 stack, a vacuum being created by its velocity and condensation. 

 This method is used in getting up steam, the blower being 



