454 STEPHENSON'S PATENT 



water cannot enter to fill it up, so much less water is forced into the boiler. In small 

 force pumps a plunger is preferable to a piston, because the barrel does not require 

 boring out, as would be the case if a piston were used ; and the packing of the stuff- 

 ing box upon the plunger is much more easily kept in order than the packing of a 

 piston. 



The additional valve in the delivery pipe acts in a similar manner to the upper valve 

 of the pump, and it is used as a security in case the other valve should get out of order 

 from any dirt getting on its seat and preventing its closing. The valves first used for 

 the feed pumps were mitre valves similar to the safety valves; but ball valves are now 

 used instead, and are found to be much superior, as they are more free and certain 

 in their action from requiring no spindle to guide them, and keep in better order. 

 The plungers of the feed pumps are sometimes attached to the cross heads, which 

 are prolonged outside of the guide blocks for the purpose, instead of being worked by 

 an arm fixed on the piston rod ; but in both plans a considerable strain is caused as 

 the pumps are so much on one side of the piston rod. To prevent this strain they 

 have been worked by eccentrics fixed upon the axle in some large engines ; in which 

 plan additional friction is produced by the eccentric, but the friction caused by the 

 strain is quite avoided, and perhaps more than compensates for it. 



THE WHEELS, FRAMING, ETC., OF THE ENGINE. 



WHEELS. The wheels are of two kinds; the two driving wheels, D ' D', which are 

 fixed on the crank axle C', are 5 feet diameter and are flat on the edge; the other four 

 wheels L'M', two of them, L', placed towards the front just behind the smoke box, 

 and the other two, M ', at the back behind the fire box, are 3 feet 6 inches diameter, 

 and have a projecting rim or flanch upon their edges which runs against the inner side 

 of the rails. Each pair of the small wheels is fixed upon an axle, L" M", as well as 

 the large wheels ; they are three inches and five eighths diameter, and the outer ends 

 project beyond the wheels, turning in brasses in the frame of the engine. Upon these 

 brasses the whole weight of the engine rests through the medium of the springs above 

 them ; and all the weight is thus suspended by springs except that of the wheels and 

 axles themselves, for the purpose of deadening the shocks that are caused by the 

 rapid motion of the engine. It is necessary for all wheels of railway carriages 

 to be fixed upon the axles and have the axles turn with them, instead of turning 

 loose upon the axles as in carriages upon common roads, in order that they may be 

 held quite steady and upright ; for if they were to get nearer together they would 

 run off the rails, though in common carriages it would be of no consequence, as they 

 would still have as firm a bearing. Also when the wheels are fixed upon the axle 



