en the faces, while, if the taper be
too little, the plug will be liable to jam, and a few times grinding will
sink it so far through the shell that the waterways will no longer
correspond. One eighth of an inch deviation from the perpendicular for
every inch in height, is a common angle for the side of the cock, which
corresponds with one quarter of an inch difference of diameter in an inch
of height; but perhaps a somewhat greater taper than this, or one third of
an inch difference in diameter for every inch of height, is a preferable
proportion. The bottom of the plug must be always kept a small distance
above the bottom of the shell, and an adequate surface must be left above
and below the waterway to prevent leakage. Cocks formed according to these
directions will be found to operate satisfactorily in practice, while they
will occasion perpetual trouble if there be any malformation.

471. _Q._--What is the best arrangement and configuration of the blow-off
cocks?

_A._--The blow-off cocks of a boiler are generally placed some distance
from the boiler; but it appears preferable that they should be placed quite
close to it, as there are no means of shutting off the water from the pipe
between the blow-off cock and the boiler, should fracture or leakage there
arise. Every boiler must be furnished with a blow-off cock of its own,
independently of the main blow-off cocks on the ship's sides, so that the
boilers may be blown off separately, and may be shut off from one another.
The preferable arrangement appears to be, to cast upon each blow-off cock a
bend for attaching the cock to the bottom of the boiler, and the plug
should stand about an inch in advance of the front of the boiler, so that
it may be removed, or re-ground, with facility. The general arrangement of
the blow-off pipes is to run a main blow-off pipe beneath the floor plates,
across the ship, at the end of the engines, and into this pipe to lead a
separate pipe, furnished with a cock, from each boiler. The main blow-off
pipe, where it penetrates the ship's side, is furnished with a cock: and in
modern steam vessels Kingston's valves are also used, which consist of a
spindle or plate valve, fitted to the exterior of the ship, so that if the
internal pipe or cock breaks, the external valve will still be operative.
Some expedient of this kind is almost necessary, as the blow-off cocks
require occasional regrinding, and the sea cocks cannot be re-ground
witho