xpense and
trouble, as from the pressure of water upon them it is difficult to prevent
them from closing with violence; and many expedients have been contrived to
mitigate the evil, of which the valve known as Harvey and West's valve has
perhaps gained the widest acceptation.

429. _Q._--Will you describe Harvey and West's pump valve?

_A._--This valve is a compromise between the equilibrium valve, of the kind
employed for admitting the steam to and from the cylinder in single acting
engines, and the common spindle valve formerly used for that purpose; and
to comprehend its action, it is necessary that the action of the
equilibrium valve, which has been already represented fig. 34, should first
be understood. This valve consists substantially of a cylinder open at both
ends, and capable of sliding upon a stationary piston fixed upon a rod the
length of the cylinder, which proceeds from the centre of the orifice the
valve is intended to close. It is clear, that when the cylinder is pressed
down until its edge rests upon the bottom of the box containing it, the
orifice of the pipe must be closed, as the steam can neither escape past
the edge of the cylinder nor between the cylinder and the piston; and it is
equally clear, that as the pressure upon the cylinder is equal all around
it, and the whole of the downward pressure is maintained by the stationary
piston, the cylinder can be raised or lowered without any further exertion
of force than is necessary to overcome the friction of the piston and of
the rod by which the cylinder is raised. Instead of the rubbing surface of
a piston, however, a conical valve face between the cylinder and piston is
employed, which is tight only when the cylinder is in its lowest position;
and there is a similar face between, the edge of the cylinder and the
bottom of the box in which it is placed. The moving part of the valve, too,
instead of being a perfect cylinder, is bulged outward in the middle, so as
to permit the steam to escape past the stationary piston when the
cylindrical part of the valve is raised. It is clear, that if such a valve
were applied to a pump, no pressure of water within the pump would suffice
to open it, neither would any pressure of water above the valve cause it to
shut with violence; and if an equilibrium valve, therefore, be used as a
pump valve at all, it must be opened and shut by mechanical means. In
Harvey and West's valves, however, the equilibrium principle