SCIENCE OF THRESHING. 125 



figure the port to the left is being closed while that 

 to the right is in communication with the exhaust 

 port. After the left port is entirely closed the piston 

 will be driven to the end of its stroke by the expan- 

 sion of the steam already admitted. 



It will be seen that it is necessary to keep the valve 

 seat and face of the valve true, so that the steam may 

 not leak through and thus cause a waste. In any 

 form of valve used (the one shown is the common 

 slide), it is necessary that the valve either move on 

 or seat itself on the seat accurately so as to prevent 

 loss of power. 



In case a slide valve is used, fashioned as that illus- 

 trated, there is a heavy pressure of steam on its upper 

 surface. This pressure is often lessened by the shaping 

 of the valve so that the steam bears on parts of the 

 valve which are on the opposite side to the parts 

 above the ports. This, of course, balances in part 

 the pressure, as only that part of the valve above the 

 port which it is covering and the surrounding valve 

 seat is not sustained by an equal and opposite 

 pressure. These forms of valve are called "balanced 

 valves." In any type of valve it is necessary to lubri- 

 cate well, and this is conveniently done by feeding in a 

 continuous supply of oil to the steam as it enters the 

 steam chest from the boiler. 



The cylinder is somewhat longer than the stroke 

 of the piston. The space between the piston head and 

 cylinder head when the latter is at the end of its 

 stroke is called the clearance; the clearance is bored 

 larger than the diameter of the cylinder and is called 



