46 POWER AND THE PLOW 



partially or wholly surrounded by a water space connected 

 with that of the boiler. In the vertical boiler the upright 

 tubes are surrounded by water to a part of their height, the 

 gases passing directly upward. This is not an economical 

 type and is little used. The greatest fuel economy is secured 

 by the return flue boiler, which takes the gases from the fire 

 box to the front of the boiler and returns them through smaller 

 flues to the rear. This type is less convenient to handle or 

 repair than the locomotive type, and cannot so easily be 

 adapted to the use of wood or straw. 



Boiler Construction 



Stringent laws in some states and provinces now prac- 

 tically force the use on plowing tractors of a boiler shell com- 

 posed of a cylindrical sheet with a single longitudinal seam. 

 The seam may be lapped or the edges of the plate may be 

 brought together and double riveted to reinforcing steel 

 straps inside and out. Steel bolts and cap screws may be and 

 sometimes are used, provided a reinforcing plate is riveted 

 on the boiler. In most late types practically no bolts are 

 used, either in the construction of the boiler or the mounting 

 of the engine and shafting upon it. Steel wing sheets are 

 riveted to the boiler for the support of these features, in place 

 of the cast iron which was formerly used for brackets and 

 various other parts not exposed to sudden change in tempera- 

 ture. The boiler on a modern plowing engine can safely be 

 run at 200 pounds pressure, though 175 pounds is usually 

 set by law as the maximum. The extreme severity of the 

 work to which the traction engine is subjected, and the neglect 

 which it suffers, are nowhere more emphasized than in the 

 extraordinary precautions which engine manufacturers are 

 required to take in constructing their boilers. 



Ordinary coal-burning "locomotive boilers may be adapted 

 for burning wood, corncobs, straw, etc. The wood-burning 

 boiler requires simply larger grate openings, if any change is 



