130 SCIENCE OF THRESHING. 



There are many varieties of valves; the description 

 of the working of one will explain the operation of 

 all. 



These figures show in skeleton the disposition of the 

 parts when the piston is at the different points of the 

 stroke. Of course, in order to give a clear view, the 

 comparative dimensions of the parts have been dis- 

 torted. 



The parts a, b, c, d, are joints or journals to allow 

 the parts to take proper positions in the different 

 views. O is the main engine shaft on which the 

 crank is secured. 



A shows the position of the crank, piston and valve 

 at the beginning of the forward stroke. The valve 

 has just started to open the steam port at the head 

 end to admit live steam to the cylinder, and has 

 uncovered the exhaust port to allow the steam from 

 the inboard end of the cylinder to escape through the 

 exhaust. 



B illustrates the position of the various parts at the 

 end of the first quarter of the revolution. The crank 

 is upright, both steam ports are fully uncovered. 

 The piston is more than half way along its stroke, on 

 account of the angularity of the connecting rod. 



C shows the end of the first half of the revolution 

 with the crank at the dead center, the piston at the 

 end of its stroke, and the valve covering the head end 

 port against live steam, and commencing to open the 

 exhaust port at that end, while the port at the inboard 

 end is just started to open to the live steam. 



