558 THE STEAM-ENGINE. 



this case that space is eighteen inches. The upper end of the vertical tube 

 G is attached by screws and a flange to a piece P containing a valve R 

 similar in all respects to the lower valve Q, and like it opening upward. A 

 screw V is introduced at the top by which access may be obtained to this 

 valve. This screw also presses on the crown of the guides of the valve, so 

 as to hold it down by regulated pressure. At the side of this upper piece P 

 is inserted a horizontal tube M connected with the end of the delivery-pipe N. 

 This latter is continued to the boiler with which it communicates at the fire- 

 box. When the plunger B is drawn out of the pump-barrel A, the spherical 

 valve Q being relieved from its downward pressure is raised, and water passes 

 from the pipe H through the valve Q into the vertical pipe G ; the lower valve 

 Q then closes and stops the return of the water. The plunger B returning 

 into the pump-barrel A then forces the water against the upper valve R and 

 drives it through the delivery-tube N, from which its return is prevented by 

 the valve R. When the delivery-tube N is filled with water throughout its 

 whole length, every stroke of the plunger will evidently drive into the boiler a 

 volume of water equal to the magnitude of a part of the plunger eighteen inches 

 in length. 



Until within the last few years, locomotive engines were supported on only 

 four wheels ; they are, however, now almost universally supported on six, the 

 driving wheels being in the middle. To give greater security to the position 

 of the engine between the rails it is usual to construct flanges on the tires of all 

 the six wheels. Mr. Stephenson,- however, has been in the practice of con- 

 structing the driving wheels without flanges, and with tires truly cylindrical, 

 depending on the flanges of the two pairs of smaller wheels to maintain the 

 engine between the rails. The wheels of the engine here described are con- 

 structed in this manner. The driving wheels D' are fixed on the cranked 

 axle C', and are five feet in diameter. The other wheels I/ M', the one being 

 placed immediately behind the smoke-box, and the other immediately behind 

 the fire-box, are each three feet six inches in diameter, and have a flange upon 

 their tires, which running on the inside of each rail keeps the engine between 

 the rails. Each pair of these small wheels, like the driving-wheels, is fixed 

 upon their axle. The axles are 3|- inches diameter, and project beyond the 

 wheels, the projecting part supporting the frame of the engine and turning in 

 brasses. Upon these brasses rest' springs, which bear the whole weight of 

 the engine. These springs having nothing between them and the road but the 

 wheels and axles intercept and equalize the sudden shocks produced by the 

 rapid motion upon the road. 



When an engine is required for the transport of very heavy loads, such as 

 those of merchandise, the adhesion of one pair of working wheels is found to 

 be insufficient, and, in such cases, one of the two pairs of wheels L' M' is 

 made of the same diameter as the wheels which are placed upon the working 

 axle, and a bar is attached to points on the outside of the wheels at equal dis- 

 tances from their centre, connecting them in such a manner that any force ap- 

 plied to make one pair of wheels revolve must necessarily impart the same 

 motion to the other pair. By such means the force of the steam is made to 

 drive both pairs of wheels and consequently a proportionally-increased adhesion 

 is obtained. 



The velocity which an engine is capable of imparting to the load which it 

 draws depends upon the rate at which the pistons are capable of being moved 

 in the cylinders. By every motion of each piston backward and forward one 

 revolution of the driving wheels is produced, and by each revolution of the 

 driving wheels, supposing them not to slip upon the rails, the load is driven 

 through a distance upon the road equal to their circumference. As the two 



