538 THE STEAM-ENGINE. 



closer together, and nearer the centre of the engine : the connecting-rods, no 

 longer attached to the wheels, are made to act upon two cranks constructed 

 upon the axle of the wheels, and placed at right angles to each other. From 

 the position of these cranks, one would always be at its dead points when the 

 otker is in full motion. The action of the steam upon them would, therefore, 

 be generally unequal ; but this would not produce the same strain as when the 

 connecting-rods are attached to points upon the exterior of the wheels, owing 

 to the cranks being constructed on the axle at points so much nearer its cen- 

 tre. By this means it was found that the working of th*e machine was more 

 even, and productive of much less strain, than in the arrangement adopted in 

 the Rocket, and the earlier engines. On the other hand, a serious disadvan- 

 tage was incurred by a double-cranked axle. The weakness necessarily ari- 

 sing from such a form of axle could only be removed by great thickness and 

 weight of metal ; and even this precaution, at first, did not prevent their occa- 

 sional fracture. '1 lie forging of thejn was, however, subsequently much im- 

 proved : the cranks, instead of being formed by bending the metal when soft- 

 ened by heat, were formed by cutting the square of the crank out of the solid 

 metal ; and now it rarely happens that one of these axles fails. 



The adoption of smaller tubes, and a greater number of them, with a view 

 more perfectly to extract the heat from the air in passing to the chimney, ren- 

 dered a more forcible draught necessary. This was accomplished by the 

 adoption of a more contracted blast-pipe leading from the eduction-pipes of 

 the cylinders, and presented up the chimney. A representation of such a blast- 

 pipe, with the two tubes leading from the cylinders, and uniting together near 

 the point, which is presented up the chimney, is given at p p in fig. 74. The 

 engine thus improved is represented in fig. 64. 



A represents the cylindrical boiler, the lower half of which is traversed by 

 tubes. They are usually from eighty to one hundred in number, and about an 

 inch and a half in diameter ; the boiler is about seven feet in length ; the fire- 

 chamber is attached to one end of it, at F, the cylinders are inserted in a 

 chamber at the other end, immediately under the chimney. The piston-rods 

 are supported in the horizontal position by guides ; and connecting-rods extend 

 from them, under the engine, to the two cranks placed on the axle of the large 

 wheels. The effects of an inequality in the road are counteracted by springs, 

 on which the engine rests ; the springs being below the axle of .the great 

 wheels, and above that of the less. The steam is applied to the cylinders, and 

 withdrawn, by means of the common sliding valves, which are worked by an 

 eccentric wheel placed on the axle of the large wheels of the carriage. The 

 motion is communicated from this eccentric wheel to the valve by sliding rods. 

 The stand is placed for the attendant at the end of the engine, next the fire- 

 place F ; and two levers L project from the end which communicate with the 

 valves, by means of rods, by which the engine is governed so as to reverse 

 the motion. 



The wheels of these engines have been commonly constructed of wood with 

 strong iron ties, furnished with flanges adapted to the rails. But Mr. Stephen- 

 son afterward substituted, in some instances, wheels of iron with hollow spokes. 

 The engine draws after it a tender carriage containing the fuel and water ; and, 

 when carrying a light load, is capable of performing the whole journey from 

 Liverpool to Manchester without a fresh supply of water. When a heavy 

 load of merchandise is drawn, it is usual to take in water at the middle of the 

 trip. 



In reviewing all that has been stated, it will be perceived that the efficiency 

 of the locomotive engines used on this railway, is mainly owing to three cir- 

 cumstances : 1st, the unlimited power of draught m the furnace, by projecting 



