Page:Popular Science Monthly Volume 24.djvu/551

Rh will exert the greatest power as yet secured from one cylinder. The stroke is fourteen feet and the diameter of the cylinder is nine feet two inches, and the engine is expected to develop eight thousand horse-power. As an illustration of the size of the engine, the wrist-pin is almost exactly the size of a flour-barrel.

We now come to the engines and boilers used for railways. The year 1832 was the beginning of our present passenger and railway system on this side of the water, and, if the engines imported in that year to run on American roads are any indication of the state of the science of steam-engineering abroad, they could not have been much in advance. At this time the engine and boiler weighed about eight tons, carried forty pounds pressure, and could make about twenty miles an hour under light load and favorable conditions. The engine of that date could not pull more than three or four times its own weight, and had to stop at stations to fill boilers, as they could not pump while running.

The speed to-day is from forty to sixty miles an hour, and the engines weigh from thirty-five to eighty tons, and draw as high as eight hundred tons of paying freight in addition to the weight of the train. To-day the pressures run from one hundred and thirty-five to two hundred pounds. The latter pressure is used in Switzerland. The automatic and continuous breaks now stop a heavy train within four hundred yards at a speed of sixty miles an hour. Recent trials show that these breaks will absorb twenty miles of speed in one minute.

In 1832 the transmission of power was by flat tumbling-rods and cast-iron shafting of great weight and little strength. To-day we have smooth, light, rapidly revolving steel or iron shafting, supplemented and aided with rubber and leather belting where the latter will serve and the former can not. Where power has to be transmitted at a great distance, wire ropes, moving at a high rate of speed, are used. Wire-rope transmission commences at the point where the belt and shafting become too long or heavy to be useful. It is much cheaper than its equivalent of shafting or belting. In fact, a long line of shafting would cost more for oil in a year than a wire rope would in fifteen.

At the Rhine-fall, in Switzerland, eight hundred horse-power is transmitted a distance of two miles to a village where fifty small manufacturing industries, situated in every conceivable position relative to the cable-line, secure power. For ten years the cable street-railway system has been in use in San Francisco. The same system, slightly modified, is being adopted in many Eastern cities.

Fifty years ago compressed air had not been successfully employed in engineering, though its application as a blast to forges is coextant with history. Sir Henry Bessemer's steel process was made possible only upon the ability of engineers to furnish air under pressure in the converter. The importance of compressed air and the part it has taken in recent engineering undertakings can not be overestimated.