receiving pipe of the front one.  This intercepting valve when in simple position, allows the high-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. The reducing valve may be stuck shut on account of being dirty or stuck on the stem of the intercepting valve.  In case the reducing valve is stuck shut, the head of the dash-pot can be taken off and the valve worked back and forth to loosen it.  The intercepting valve should be liberally oiled just before starting and occasionally during long runs to keep it free from sticking.