n on the point of the tool. A boring mill, of which the speed
may be varied from one turn in six minutes to twenty-five turns in one
minute, will be suitable for all ordinary wants that can occur in practice.

703. _Q._--Are there any precautions necessary to be observed in order that
the boring may be truly effected?

_A._--In fixing a cylinder into the boring mill, great care must be taken
that it is not screwed down unequally; and indeed it will be impossible to
bore a large cylinder in a horizontal mill without being oval, unless the
cylinder be carefully gauged when standing on end, and be set up by screws
when laid in the mill until it again assumes its original form. A large
cylinder will inevitably become oval if laid upon its side; and if while
under the tension due to its own weight it be bored round, it will become
oval again when set upon end. If the bottom be cast in, the cylinder will
be probably found to be round at one end and oval at the other, unless a
vertical boring mill be employed, or the precautions here suggested be
adopted.

704. _Q._--Does the boring tool make the cylinder sufficiently smooth for
the reception of the piston?

_A._--Many engine makers give no other finish to their cylinders; but
Messrs. Penn grind their cylinders after they are bored, by laying them on
their side, and rubbing a piece of lead, with a cross iron handle like that
of a rolling stone, and smeared with emery and oil, backward and forward--
the cylinder being gradually turned round so as to subject every part
successively to the operation. The lead by which this grinding is
accomplished is cast in the Cylinder, whereby it is formed of the right
curve; but the part of the cylinder in which it is cast should be
previously heated by a hot iron, else the metal may be cracked by the
sudden heat.

705. _Q._--How are the parts of a piston fitted together so as to be
perfectly steam tight?

_A._--The old practice was to depend chiefly upon grinding as the means of
making the rings tight upon the piston or upon one another; but scraping is
now chiefly relied on. Some makers, however, finish their steam surfaces by
grinding them with powdered Turkey stone and oil. A slight grinding, or
polishing, with powdered Turkey stone and oil, appears to be expedient in
ordinary cases, and may be conveniently accomplished by setting the piston
on a revolving table, and holding the ring stationary by a cross piece of
wood while the tab