An “eccentric” which is really a form of crank, drives the valve to and fro, the eccentric being fastened to the crankshaft. The full pressure of the steam forces the D valve down on its seat, and as the valve is of considerable size, this pressure causes much friction and power loss. In some engines a “balanced” valve is used in which the pressure on the valve is balanced by an equal pressure that acts on the under side of the valve face. Balanced or unbalanced, the function of the slide is to alternate the flow of steam in the two ends of the cylinder. Steam is prevented from passing the piston into the opposite end of the cylinder by elastic rings placed in grooves on the piston which are known as “piston rings.” Being thin and elastic these rings instantly conform with any irregularity of the piston bore and effectually stop the flow of steam past them. At the point where the reciprocating piston rod R passes through the cylinder, a steam tight joint is made by the “stuffing box” or gland H. The space between the inner walls of the stuffing box and the piston rod are either filled with some description of fibrous packing or a metallic packing that fits around the rod in the same manner that the piston rings fit in the bore of the cylinder. The packing is arranged around the valve rod VR in the same manner. As the piston, piston rod, and valve slide on their respective surfaces with considerable pressure it is absolutely necessary that these parts receive ample lubrication. In practically all engines the oil is taken into the cylinder with the steam in the form of drops, the oil being measured out by a sight feed lubricator that is tapped into the steam supply pipe. In this device, the oil from the lubricator reservoir is fed through a regulating needle valve, drop by drop, up through a gauge glass so that the engineer can tell the amount of oil that he is feeding. The body of the lubricator is filled with condensed water up to the level of the outlet through which the oil passes into the cylinder, and the entire lubricator, reservoir and all is under boiler pressure at all points. The oil regulating valve is placed at the bottom of the lubricator, and as oil is lighter than water, it floats up from the valve to the level of the outlet, through the gauge glass, and from the outlet level floats out into the steam pipe and mixes with the steam. By floating the oil in this manner, the engineer can see every drop that is fed. (149) Expansion of Steam.