manner, but so much smaller as to leave a space between the shell and the
core equal to the thickness of the cylinder, and into this space the melted
metal is poured. Whatever nozzles or projections are required upon the
cylinder, must be formed by means of wooden patterns, which are built into
the shell, and subsequently withdrawn; but where a number of cylinders of
the same kind are required, it is advisable to make these patterns of iron,
which will not be liable to warp or twist while the loam is being dried.
Before the iron is cast into the mould, the interior of the mould must be
covered with finely powdered charcoal--or blackening, as it is technically
termed; and the secret of making finely skinned castings lies in using
plenty of blackening. In loam and dry sand castings the charcoal should be
mixed with thick clay water, and applied until it is an eighth of an inch
thick, or more; the surface should be then very carefully smoothed or
sleeked, and if the metal has been judiciously mixed, and the mould
thoroughly dried, the casting is sure to be a fine one. Dry sand and loam
castings should be, as much as possible, made in boxes; the moulds may
thereby be more rapidly and more effectually dried, and better castings
will be got with a less expense.

702. _Q._--Will you explain the next operation which a cylinder undergoes?

_A._--The next stage is the boring; and in boring cylinders of 74 inches
diameter, the boring bar must move so as to make one revolution in about
4-1/2 minutes, at which speed the cutters will move at the rate of about
5 feet per minute. In boring brass, the speed must be slower; the common
rate at which the tool moves in boring brass air pumps is about 3 feet per
minute. If this speed be materially exceeded the tool will be spoiled, and
the pump made taper. The speed proper for boring a cylinder will answer for
boring the brass air pump of the same engine. A brass air pump of 36-1/2
inches diameter requires the bar to make one turn in about three minutes,
which is also the speed proper for a cylinder 60 inches in diameter. To
bore a brass air pump 36-1/2 inches in diameter requires a week, an iron
one requires 48 hours, and a copper one 24 hours. In turning a malleable
iron shaft 12-3/4 inches in diameter, the shaft should make about five
turns per minute, which is equivalent to a speed in the tool of about 16
feet per minute; but this speed may be exceeded if soap and water be
plentifully ru