port. When the crank pin has reached the back dead centre, the angle of the eccentric rod will be in reverse direction from its appearance in the cut, and the centre of the valve will be the same distance from the middle of its seat as is shown, but to the left, thus providing the lead opening to the back, or right-hand, admission port. If lever R should be moved further forward, the increased angle of the link would give a longer forward-and-back motion to the link-block and a lengthened travel of the valve; hooking up will have the opposite effect, until with lever R in mid-gear the only motion given to the valve would be its short motion equal to twice the lap plus lead. With the engine standing as shown in the cut, if lever R should be thrown into back gear, it is plain that, as the eccentric's movement is upward as the wheel turns backward, the link-block will be moved back and up, and the valve will again be moved in the right direction.

With a valve of inside admission, the eccentric must be located 180 degrees from the crank pin-just opposite its present position-and this fixes the valve's advance and prospective travel in the opposite direction to the design here shown.

In this style of valve gear (as in Walschaerts') the lead is constant; proven when the crank pins are on either dead centre, by shifting the reverse lever from

one full gear position to the opposite extreme by which the valve will not be moved.

THE HOBART-ALLFREE CYLINDERS,
VALVES AND VALVE GEAR

Cylinders and Valves.-The Hobart-Allfree cylinder and piston, valves and valve chambers, are shown in Figs. 42, 43, and 44. The advantages claimed in their use are increased pulling power and greater speed, economy in steam and fuel, cheaper cost of maintenance and more mileage secured from the engine, due to-reduced heat losses (short ports, and the insulation of the live steam); less steam used for a given cut-off (waste spaces in cylinders reduced to a minimum); higher ratio of expansion (as the result of dividing the steam distribution between two valves); perfect exhaust (for the same reason, and giving a better draft on the fire, with reduced back pressure); reduced "negative" work (very late exhaust closure--only enough compression to provide for the cushioning of the piston); together with an increased turning effort on the drivers.

Of the four valve events embraced in the proper steam distribution in the cylinders of an engine, the single valve of any style can perform three perfectly: admission, cut-off, and release, but the fourth-known as the closure or compression event, is wrong. If the

valve is provided with a sufficient amount of exhaust lap to obtain economic expansion by delaying the release, the exhaust closure becomes excessively early, and an excessive clearance space must be provided, equal to fully 15% of the piston displacement; this entails such a loss that a compromise is generally effected by reducing the exhaust lap, making the release earlier but delaying the closure to a point where not to exceed 10% of clearance space need be provided. This requires a valve having its exhaust edges about equal to the exhaust edges of the ports, generally designated as "line and line," or, if the valve is cut a little shorter, giving about -inch exhaust clearance, the clearance or waste spaces may be reduced to about 8% of the piston displacement. Any further reduction of exhaust lap would result in a greater loss through an early release than would be gained through a reduction of clearance.

In the Allfree system a single valve correctly controls admission, cut-off, and release, the same as in the standard engine, except that a sufficient amount of exhaust lap is used to carry the steam to a point that will give a greater expansion; then, to avoid an early closure, a small piston valve, called the compression valve, is introduced through a section of the ports beneath and to one side of the main valve, and has only the function of controlling the compression and providing