886 AGRICULTURAL ENGINEERING 



by the valve mechanism, or valve gear, as it is called. The 

 piston is fitted with rings which expand against the walls of 

 the cylinder, making a gas-tight fit. The power developed 

 by the engine is proportional to the travel of the piston in one 

 minute and the average pressure of the steam on its face. 



Compound Engines. The compound engine has two 

 cylinders. The steam is admitted first into the smaller one 

 and allowed to expand to a certain pressure, and then it passes 

 to the second, where it expands more fully. The compound 

 engine enables the cylinders to be maintained at more nearly 



4! 



Fig. 248. A sectional view of the cylinders and steam chest of 

 a compound engine. 



the temperature of the steam. As steam expands, it cools; 

 and when fresh steam is admitted after the expansion of a 

 cylinderful, some of it condenses, losing part of its power. 

 Compound engines also tend to equalize the pressure of the 

 steam on the piston throughout the stroke, giving a steadier 

 motion and lowering the stress upon the working parts. 



The Double Engine. Many traction engines are pro- 

 vided with two cylinders, making a double engine. The 

 cranks are on the same shaft, but are located at an angle of 

 90 degrees with each other, so that at no time can both cranks 



