Page:Popular Science Monthly Volume 12.djvu/293

Rh valve is, outside inch, inside  inch. The eccentrics have a throw of 4 inches. The standard freight-engine has six driving-wheels, 54 inches in diameter. The steam-cylinders are eighteen inches in diameter, stroke twenty-two inches, grate-surface 14.8 square feet, heating-surface 1,096 feet. It weighs 68,500 pounds, of which 48,000 are on the drivers and 20,500 on the truck. The boiler is nearly of the same dimensions as that of the passenger-engine, but the tubes are 2 inches in diameter, twelve feet 9 inches long, and 119 in number. The stack is eighteen inches in diameter. The pump is 2¼ inches in diameter, and has a stroke of twenty-two inches. The valve has inch inside lap,  inch outside. The former takes a train of five cars up an average grade of ninety feet to the mile. The latter is attached to a train of eleven cars. On a grade of fifty feet to the mile, the former takes seven and the latter seventeen cars. Tank-engines for very heavy work, such as on grades of 320 feet to the mile, which are found on some of the railroads where gradients are very steep, have five pairs of coupled driving-wheels, and are not fitted with trucks. Such engines have, usually, steam-cylinders about twenty inches in diameter and two feet stroke of piston. Their grates have an area of fifteen or sixteen square feet, and the heating-surface has an area of 1,400 to 1,500 square feet. Engines of this class, weighing fifty tons, have hauled 110 tons up the heaviest grades of the Pennsylvania Railroad at the rate of five miles an hour. Steam-pressure is carried at from 125 to 150 pounds on the square inch.

70. A train weighing 150 tons is drawn by an express-engine (Fig. 43)



at the speed of sixty miles an hour, the engine developing about 800 horse-power. An engine drawing a light train has been known to make about one hundred miles in one hundred minutes, which speed may be taken as representing the maximum for the best modern engines on the best existing roads.