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ARTICLE VIII.
OF THE SUPPLY PUMP.
WE supply the boiler with water by a small forcing pump, wrought by the engine, which requires about one thousandth part of the power to work it, to force a little water into the boiler at every stroke, and we experience great loss of power by using cold water as a supply; for although it lowers the heat in the boiler but litfie, yet as it has been shown, that a small increase of heat, say 30 degrees, doubles the power, so a small diminution of heat, say 30 degrees, reduces the power to one half; say from 120 pounds to 60 pounds to the inch, (see article 3.)
Therefore we construct a strong, small vessel called the
supply boiler, to be heated either by the steam passing through it, after
leaving the engine, or by passing the flue of the furnace through or under
it, after leaving the boiler. The supply pump brings the water up from the
well or stream, or if one be used, out of the condenser, forcing it into
the supply boiler, which it keeps always full, out of which it passes by
a small pipe into the principal boiler, and may be thus heated by the smoke
flue, to the same degree with the water in the boiler before it enters.
This may appear incredible, until we consider that the steam which works
the engine, carries off the heat from the boiler, which is not the case
with the supply boiler, from which no steam escapes. And as we may suppose
that cold water receives heat more freely than hot, we sometimes
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make our boilers in several separate parts, passing the
flue of the furnace through them all, and forcing our supply water into
that part farthest from the fire, to pass from one to the other by small
connecting pipes meeting the fire; on these principles we obtain a greater
quantity of heat from the fire into the water.