82 POWER AND THE PLOW 



To prevent preignition, and at the same time make the fuel 

 do the increased work of which it is capable at the higher 

 compression, the tendency is now to adopt a form of car- 

 bureter which will automatically vary the quality as well as 

 the quantity of the mixture under different loads. This type 

 of carbureter, intimately connected with a positively driven 

 governor, has given excellent regulation, approached only by 

 first-class steam engines in large stationary plants. The nu- 

 merous adjustments are being dispensed with and the simple 

 expedient of controlling the vacuum in the carbureter has been 

 adopted. This is accomplished by varying the relative pro- 

 portions of the passages from the atmosphere to the carbureter, 

 thence to the cylinder. Substantial sliding plates directly 

 connected with the governor displace many of the delicate 

 parts which formerly handicapped the tractor in rough work. 

 The uniform conditions in the cylinder, which are thus ob- 

 tained, allow the use of gasoline, kerosene, or even heavier 

 oils under a wide range of conditions. 



It has been stated by eminent authority that some heating 

 apparatus is necessary in order to vaporize kerosene success- 

 fully. However, at the present time, thousands of tractors are 

 working in the field on gasoline, kerosene, or distillate without 

 changing the carbureter and without requiring heat for the 

 action of the latter. Both "hit-and-miss" and throttling 

 governors are employed on kerosene engines, in spite of other 

 high authority to the effect that a change must be taken at 

 every cycle to prevent cooling and loss of efficiency. 



FUEL SUPPLY 



In some tractors the fuel is delivered to the carbureter by 

 gravity, the carbureter being required to control the supply 

 to the cylinder against the pressure of the liquid in the tank. 

 This method does away with fuel pumps and additional piping, 

 but is not positive in its action, as the pressure varies with the 



