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.

38. Q. Why does the Mallet compound have more power when working simple
than compound?

A. If a starting valve is used to admit live steam to the receiver pipe
and thence to the low-pressure engine, this gives a higher pressure to
the low-pressure cylinders. If an intercepting valve is used, the open
emergency exhaust valve allows exhaust steam from the rear engine to go
direct to the stack; this takes away the back pressure of the receiver
steam from the high-pressure pistons, about 30 per cent. of the boiler
pressure, and thus adds to the power of the rear engine. The reducing
valve when feeding live steam gives about 40 per cent. of boiler
pressure to the low-pressure engine instead of the 30 per cent. it gets
from the receiver; the added power of both engines working simple is
about 20 per cent. over the compound operation.

39. Q. What is the duty of the by-pass valves on
the sides of the low-pressure cylinders? Should they be kept clean of
gum and grit?

A. These valves are connected to the steam ports at each end of the
cylinders and open to allow air and steam to pass from one end of the
cylinder to the other; away from the moving piston when the engine is