valve by moving the reverse lever to the center notch.  This should cover the ports and if the valve is tight the blow will stop.  To test the low-pressure piston, place the engine in the same position with wheels blocked, starting valve open, back indicator plug out; when throttle is opened, the leaky packing will be shown by steam issuing from the plug opening.  If uncertain, the valve can be tested by bringing reverse lever to the center of quadrant, which will spot valve over port and if it is tight the blow will stop.  In any compound engine 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