Transportation on Land 293 



diameter and have a piston stroke of thirty-two inches. Steam 

 is supplied at a gauge pressure of 210 pounds, and utilized in 

 expansion at the ratio of 1 to 2 (compare pages 294, 309 on 

 pressure and expansion in separate cylinders) . The engine per- 

 forms the work of three engines of the large freight service type. 



Comparison with Stephenson's locomotive. Compare these 

 data with what we know of the first locomotive (Fig. 91), in 

 which Stephenson, about the year 1815, worked out success- 

 fully the direct transmission of power to the driving wheels. 

 The engine with its tender weighed 36,000 pounds ; it hauled 

 a train of ten cars or wagons, weighing with their load about 

 50 tons, at a speed of five miles an hour, or a load of 70 tons 

 at a much less speed. It had four drive wheels, each two feet 

 in diameter. 



126. The operation of the steam engine. The principal 

 parts of the steam engine cannot be illustrated perfectly in a 

 diagram ; they should be studied with the aid of a model and 

 by observation of an actual engine. The important parts, 

 besides the boiler, are the steam chest, the slide valve, the 

 exhaust port, the cylinder, and the piston, which is connected 

 by a rod to the crank shaft of a stationary engine or to the 

 driving wheels of a locomotive. 



The operation of the engine is suggested in the diagrams of 

 Fig. 93. Steam is admitted, as in diagram 1, through pipes 

 by a valve which is under control of the engineer, into the 

 steam chest C, and then into the cylinder through the right end 

 port B. The high pressure steam drives the piston toward the 

 left, and at the same time forces the used steam out through 

 port A and the exhaust port E. 



In diagram 2, the slide valve is closed when the piston stroke 

 is about one half complete, to allow the steam by its continued 

 expansion to complete the stroke. Just before the stroke is 

 completed, the valve A is opened to admit steam to form a 

 cushion, which prevents pounding and also furnishes enough 

 live steam to begin the return stroke. 



