THE STEAM TRACTOR 51 



at 212 F. The amount thus evaporated, compared with the 

 heat units supplied in the fuel, determines the boiler efficiency. 

 Another method of rating is by the area of heating surface. 

 The heating surface includes all parts exposed to heat on one 

 side and water on the other. It includes the crown sheet, 

 the insides of all flues, the water legs, and the part of the tube 

 sheet exposed to heat. It is customary to allow one horse- 

 power for from 11.5 to 14 square feet of heating surface. This, 

 however, gives a rating too low for the majority of traction 

 engine boilers, on account of the forced draft commonly 

 employed. 



THE TRACTOR ENGINE 



Types of Engines 



The majority of engines are of the simple, non-condensing 

 type. In other words, the steam is expanded but once, and 

 the exhaust steam is not condensed so as to retain its heat. 

 They are not economical, therefore, as compared with those 

 found in large stationary plants. In the compound engine 

 the steam is usually superheated by passing it back through 

 pipes in the steam space and fire box. It is admitted first to 

 a small cylinder and again expanded in a larger one. Both 

 cylinders work through a shorter range of temperature; less 

 radiating surface is exposed to the high pressure steam, and 

 much less material is required to make the small cylinder 

 sufficiently strong for safety. There are very few compound 

 engines on plowing tractors, though both tandem and cross- 

 compound engines are used with good results. In the former 

 the high and low pressure cylinders are placed end to end, 

 while in the latter they are side by side. Some cross-compound 

 engines may be converted at will into double simple engines 

 for starting a heavy load or moving it at a slow speed. 



The larger and more powerful simple engines are usually 

 equipped with two cylinders for plowing purposes. For 



