a blow past the high-pressure packing tends to increase
the pressure in the low-pressure cylinder. A blow past the low-pressure
packing can always be heard at the exhaust, and is usually on both
forward and back strokes, while a blow past the by-pass valves or valve
bushings occurs at a certain part of a complete revolution only.

31. Q. In case it was necessary to disconnect on one side of a compound
engine, how would you cover ports and hold valves in position?

A. The easiest way is to clamp the valve stem to hold valve in mid
position; this should cover all ports. It may be necessary to take off
head of piston valve chest and block in there.

32. Q. Is it a disadvantage to work a compound engine in short cut-off?
Why?

A. Yes. If cut-off is too short the proper proportion of steam passing
the throttle will not get to the low-pressure cylinder. The work should
be divided between the two cylinders on same side.

33. Q. In what way do the Mallet or articulated compounds differ from
other steam locomotives in the distribution of the steam?

A. Mallet compounds have two separate and complete engines under one
boiler. The rear engine has a rigid connection to the back end of the
boiler; this engine works boiler steam direct the same as a simple
locomotive. Under the front end of the boiler is another engine so
constructed that the entire front engine can move from side to side
under the boiler, having a hinged connection at the front end of the
rear engine to allow the locomotive to pass curves more easily. The
front engine takes the exhaust steam from the rear engine through a
flexible pipe or receiver and works it through a larger set of cylinders
and thus compounds the steam. From the low-pressure cylinders the steam
is exhausted to the atmosphere through the stack.

34. Q. How do you get the use of both engines when starting a train?

A. To get steam into the low-pressure cylinders before the high-pressure
engine has exhausted, some types of the Mallet compound have a live
steam pipe with a valve in the cab to admit boiler steam to the receiver
pipe and thus get the use of the front engine in starting a train. The
American Locomotive Company articulated compounds have an intercepting
valve similar to the one used in the Richmond cross compound, located
between the exhaust passage of the rear engine and the flexible
receiving pipe of the front one. This intercepting valve when in SIMPLE
position, allows the hig