produced the " half-crank " axle, whose outer cheek was omitted and whose "wrist" was fixed in a spoke of the wheel. This strengthened the axles and allowed the boiler to be lower down and larger in diameter, and also permitted the driving-axle to be back of the fire-box, while the connecting-rods passed by the side of the fire-box and took bold inside the wheel. This involved putting the cylinders outside the smoke box. Among the last engines on which the half-crank axle was used were those for the Erie road in 1849. For some years after 1833, Baldwin built his engines with cylindrical pedestals for the driving- and tender-wheels. In 1852 the driving-boxes were made with a slot in the line of the vertical bearing of the journal, to distribute the wear more evenly. The straight driving-axle needs no illustration; the crank form, which answers for inside-cylinder engines, is shown in Figure 14 (pi. ioo).
Locomotive locomotive valves were of the slide pattern (plain "D"), and "book-gear." Some of the early engines had the eccentric outside the journals and wheels. It was not long before independent cut-off valves were devised. Rogers as early as 1843 gave much thought to the subject of working steam expansively. Many of the engines of 1848 had independent cut-off valves, such as are now again coming into use. In 1852, Baldwin produced a variable cut-off with a riding cut-off valve, which fitted almost tight to the main valve below and to the sides of the chest.
Baldwin introduced what is called the half-stroke cut-off, in which the chest was separated into an upper and a lower part by a plate on which rode a separate cut-off valve. About 1868 the Bristol roller-slide valve was introduced, the pressure of the slide being taken by anti-friction rollers, but it was abandoned after extensive trial.
About 1882 the Allen valve was introduced, both balanced and unbal anced; as now balanced by Richardson, it is doing good service. Its peculiarity consists in a supplementary port cored out in the valve itself, so as to admit steam into the steam-port both inside and outside the lip, thus requiring but one-half the valve-travel that is needed where steam is let in only at the outer edge of the lip.
first employed the link-motion in 1832, and the Stephensons used it at once in England, but it was not until 1849 that it was adopted in the United States, where its introduction was violently opposed. Rogers used the suspended link (pi. fig. 1) in 1849, and in 185o the shifting link, which had the lifting-shaft below the link (fig. 2); but the front axles of some ten-wheeled engines coming in the way of the rocking-shaft, lie put the shaft above the link. As early as 1854 there
was used by Rogers a combination of independent graduated cut-off valve with the link (fig. 3). For some ten-wheeled engines Hudson made curved eccentric-rods, so as to clear the axles. In 1886, Uhry S Luttgens applied a supplementary cam-motion to the link to give greater steam port opening and to retard the exhaust without affecting the compression. At first, counterweights were used for balancing the weight of the shifting link, but afterward leaf-springs were used, then helical and volute.
great and increasing weight and speed of railway trains have rendered necessary some means of bringing them rapidly to a standstill from a high rate of speed. This is usually accomplished by increasing the friction of the wheels throughout the train by pressing against the treads of the wheels "shoes" composed of iron and conform ing closely to the periphery of the wheels (at the same time shutting off steam and in some cases reversing the engine). In this way the resistance to the train's ongoing is greatly augmented, and a portion of the train's momentum is absorbed in the production of heat at the contacting sur faces, and in the removal of portions of iron from the brakes and of iron or steel, as the case may be, from the wheel. The proper poiuts at which to apply the brake-power to the wheels (supposing these to be the places at which the friction was created) would be at the sides of the tires, or upon the peripheries of the friction-drums, concentric with the wheels and either made in one therewith or fastened to the same axles. By this means there would be obviated the reduction of diameter of the wheel—an objection able feature, in that maintenance of the original diameter is desirable and that the material rubbed off is expensive, also that when the tire gets worn down to a certain thickness either the whole wheel must be discarded (in the case of cast-iron wheels) or the old tire must be removed and a new one put on.
But the proper place of application of the brake-shoe is the rail itself. Absolute prevention of rotation of the wheel at once produces "skid ding," which causes flat spots on the wheel-tread, makes the riding rough, and endangers the train. Any lessening of the peripheral speed of the wheels to a rate less than the lineal speed of the train upon the rail, also causes injurious action and wear of wheel or rail or both. But at present the crude method of brake-application to the wheel-treads is adopted, and the devices for effecting this are most prompt and effective.