HANGING ROCK IRON REGION.              7

cylinders. The furnaces are usually worked with open hearth,
and most of them use but one tuyere, entering at the side.
The hearths are generally constructed of sandstone, of which
there is an abundance of good quality on most of the furnace
estates. Formerly the inwall or lining was also generally of
sandstone, but of late years the use of fire-brick for that pur-
pose has become more common.
Most of the furnaces are without either pressure gauge or
pyrometer, so that the pressure and temperature of the blast
are not known with any accuracy, the general rule being to
heat the blast as high as possible with the ovens at command,
and to keep the pressure as great as the steam will allow,
varying it, of course, according to the necessities of the furnace
working. It is probable that the general pressure of blast
does not vary much from two and a half to three pounds.
The following table gives the principal dimensions and other
details of the charcoal furnaces in this portion of Kentucky:
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