The _piston_ is a circular disc working back and forth in the cylinder. It is usually a hollow casting, and to make it fit the cylinder steam tight, it is supplied on its circumference with _piston rings_. These are made of slightly larger diameter than the piston, and serve as springs against the sides of the cylinder. The _follower plate_ and bolts cover the piston rings on the piston head and hold them in place.
[Illustration: CONNECTING ROD AND CROSS-HEAD.
The _piston rod_ is of wrought iron or steel, and is fitted firmly and rigidly into the piston at one end. It runs from the piston through one head of the cylinder, passing through a steam-tight "stuffing box." One end of the piston rod is attached to the cross-head.
The _cross-head_ works between _guides_, and has _shoes_ above and below. It is practically a joint, necessary in converting straight back and forth motion into rotary. The cross-head itself works straight back and forth, just as the piston does, which is fastened firmly to one end. At the other end is attached the _connecting rod_, which works on a bearing in the cross-head, called the _wrist pin_, or cross-head pin.
The _connecting rod_ is wrought iron or steel, working at one end on the bearing known as the wrist pin, and on the other on a bearing called the _crank pin_.
The _crank_ is a short lever which transmits the power from the connecting rod to the _crank shaft_. It may also be a disc, called the _crank disc_.
Let us now return to the steam cylinder itself.
The steam leaves the boiler through a pipe leading from the top of the steam dome, and is let on or cut off by the _throttle_ valve, which is usually opened and closed by some sort of lever handle. It passes on to the _Steam-chest_, usually a part of the same casting as the cylinder. It has a cover called the _steam-chest cover_, which is securely bolted in place.
The _steam valve_, usually spoken of simply as the _valve_, serves to admit the steam alternately to each end of the cylinder in such a manner that it works the piston back and forth.
There are many kinds of valves, the simplest (shown in the diagram) being the D-valve. It slides back and forth on the bottom of the steam-chest, which is called the _valve seat_, and alternately opens and closes the two _steam ports_, which are long, narrow passages through which the steam enters the cylinder, first through one port to one end, then through the other port to the other end. The exhaust steam also passes out at these same ports.