THE GROWTH OF THE STEAM-ENGINE. 



279 



valve is, outside inch, inside -fa inch. The eccentrics have a throw of 

 4i inches. The standard freight-engine has six driving-wheels, 54 

 inches in diameter. The steam-cylinders are eighteen inches in diame- 

 ter, 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 T 9 F inches long, and 119 in number. The 

 stack is eighteen inches in diameter. The pump is 2 inches in diam- 

 eter, and has a stroke of twenty-two inches. The valve has inch 

 inside lap, -fa 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 Rail- 

 road 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. 



Fig. 43. Standard Passenger and Express Engine, 18T8. 



43) at the speed of sixty miles an hour, the engine developing about 

 800 horse-power. 1 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. 



1 Nearly equivalent to the actual power of 1,200 horses. 



