h-pressure cylinders of the rear engine to
exhaust directly to the stack instead of into the receiver, and feeds
boiler steam at a reduced pressure into the receiver pipe for the
low-pressure cylinders without giving any back pressure on the
high-pressure pistons. This increases the power of the complete
locomotive about 20 per cent. When in compound position, the
intercepting valve cuts off the supply of live steam to the receiver
pipe and forces the exhaust steam to go to the low-pressure engine
ahead.

35. Q. How is the American articulated compound changed from compound to
simple, and back to compound again?

A. To work the locomotive simple, place the handle of operating valve in
the cab to point toward the rear. This admits steam against the piston
that operates the emergency exhaust valve and opens it. Exhaust steam
from the high-pressure engine can pass to the exhaust nozzle instead of
to the low-pressure engine. The intercepting valve then moves over so
that live steam reduced to 40 per cent. of boiler pressure goes through
the receiver pipe to the low-pressure engine. To work compound, place
the handle of the operating valve to point forward. This will exhaust
the steam, holding the emergency exhaust valve open; a spring and the
pressure of the steam exhausted from the rear engine will close the
emergency exhaust valve and build up a pressure against the
intercepting valve that will open it so exhaust steam from the rear
engine will go to the forward one and at the same movement close the
reducing valve so no more live steam goes to the receiver.

36. Q. When is it necessary to use the operating valve to change the
locomotive from compound to simple, or from simple to compound?

A. When giving the engines steam to start, the intercepting valve should
automatically go to simple position until exhaust steam from the rear
engine builds up a receiver pressure that shifts the valve to compound;
if it does not, use the operating valve. When moving less than four
miles an hour or when about to stall on a grade, set the engines working
simple; changing to compound when the danger of stalling is over or the
speed is more than four miles an hour. If there is no intercepting valve
to furnish live steam to the forward engine, open the starting valve to
admit live steam to the receiver pipe and low-pressure engine.

37. Q. If in starting the locomotive the forward engine does not take
steam, what is the trouble?

A.