em I found to be the largeness of the space left
between the valve plates in this class of pumps, and out of which there is
nothing to press the air or water which may be lying there. It consequently
happens, that if there be the slightest leakage of air into the pump, this
air is merely compressed, and not expelled, by the advance of the air pump
piston. It expands again to its former bulk on the return of the pump
piston, and prevents the water from entering until there is such an
accumulation of pressure in the condenser as forces the water into the
pump, when the air being expelled by the water, causes a good vacuum to be
momentarily formed in the pump when it gorges itself by taking a sudden
gulp of water. So soon, however, as the pressure falls in the condenser and
some more air leaks into the pump, the former imperfect action recurs and
is again redressed in the same violent manner.

465. _Q._--Is this irregular action of the pump the cause of the imperfect
vacuum?

_A._--It is one cause. Sometimes one end of the pump will alone draw and
the other end will be inoperative, although it is equally open to the
condenser, and this will chiefly take place at the stuffing box end, where
a leakage of air is more likely to occur. I find, however, that even when
both ends of the pump are acting equally and there is no leakage of air at
all, the vacuum maintained by a double acting horizontal pump with india
rubber valves, is not so good as that maintained by a single acting pump of
the kind usual in old engines.

466. _Q._--Will you specify more precisely what were the results you
obtained?

_A._--When the vacuum pan was exhausted by the pumps without any boiling
being carried on in the pan, but only a little cold water being let into
it, and also into the pumps to enable them to act in their best manner, it
was found that whereas with the old pump a vacuum of 114 on the sugar
boiler's gauge could be readily obtained, equal to about 29-1/2 inches of
mercury, the lowest that could possibly be got with the new horizontal pump
was 122 degrees of the sugar boiler's gauge, or 29 inches of mercury, and
to get that the engine must not go faster than 10 or 12 strokes per minute.
The proper speed of the engine was 75 strokes per minute, but if allowed to
go at that speed the vacuum fell to 130 of the sugar maker's gauge, or
28-1/2 inches of mercury. When the steam was let into the worms of the pan
so as to boil the water in it,