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, Mat 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