[Illustration: 900]
1. The following figures of a felspar and flint mill are taken from
plans of apparatus lately constructed by Mr. Hall of Dartford, and
erected by him in the royal manufactory of Sèvres. There are two similar
sets of apparatus, _fig._ 900., which may be employed together or in
succession; composed each of an elevated tub A, and of three successive
vats of reception A´, and two behind it, whose top edges are upon a
lower level than the bottom of the casks A, A, to allow of the liquid
running out of them with a sufficient slope. A proper charge of kaolin
is first put into the cask A, then water is gradually run into it by the
gutter adapted to the stopcock _a_, after which the mixture is agitated
powerfully in every direction by hand with the stirring-bar, which is
hung within a hole in the ceiling, and has at its upper end a small
tin-plate funnel to prevent dirt or rust from dropping down into the
clay. The stirrer may be raised or lowered so as to touch any part of
the cask. The semi-fluid mass is left to settle for a few minutes, and
then the finer argillaceous pap is run off by the stopcock _a´_, placed
a little above the gritty deposit, into the zinc pipe which conveys it
into one of the tubs A´; but as this semi-liquid matter may still
contain some granular substances, it must be passed through a sieve
before it is admitted into the tub. There is, therefore, at the spot
upon the tub where the zinc pipe terminates, a wire-cloth sieve, of an
extremely close texture, to receive the liquid paste. This sieve is
shaken upon its support, in order to make it discharge the washed
argillaceous kaolin. After the clay has subsided, the water is drawn off
from its surface by a zinc syphon. The vats A´ have covers, to protect
their contents from dust. In the pottery factories of England, the
agitation is produced by machinery, instead of the hand. A vertical
shaft, with horizontal or oblique paddles, is made to revolve in the
vats for this purpose.
[Illustration: 901]
_The small triturating mill_ is represented in _fig._ 901. There are
three similar grinding-tubs on the same line. The details of the
construction are shown in _figs._ 902, 903, where it is seen to consist
principally of a revolving millstone B (_fig._ 902.), of a fast or
sleeper millstone B´, and of a vat C, hooped with iron, with its top
raised above the upper millstone. The lower block of hornstone rests
upon a very firm basis, _b´_; it is surrounded immediately by the strong
wooden circle _c_, which slopes out funnel-wise above, in order to throw
back the earthy matters as they are pushed up by the attrition of the
stones. That piece is hollowed out, partially to admit the key C,
opposite to which is the faucet and spigot _c´_, for emptying the tub.
When one operation is completed, the key C is lifted out by means of a
peg put into the holes at its top; the spigot is then drawn, and the
thin paste is run out into vats. The upper grindstone, B _d_, like the
lower one, is about two feet in diameter, and must be cut in a peculiar
manner. At first there is scooped out a hollowing in the form of a
sector, denoted by _d e f_, _fig._ 903.; the arc _d f_ is about
one-sixth of the circumference, so that the vacuity of the turning
grindstone is one-sixth of its surface; moreover, the stone must be
channelled, in order to grind or crush the hard gritty substances. For
this purpose, a wedge-shaped groove _d e g_, about an inch and a quarter
deep, is made on its under face, whereby the stone, as it turns in the
direction indicated by the arrow, acts with this inclined plane upon all
the particles in its course, crushing them and forcing them in between
the stones, till they be triturated to an impalpable powder. When the
grindstone wears unequally on its lower surface, it is useful to trace
upon it little furrows, proceeding from the centre to the circumference,
like those shown by the dotted lines _e´ e´´_. It must, moreover, be
indented with rough points by the hammer.
[Illustration: 902 903]
The turning hornstone-block is set in motion by the vertical shaft H,
which is fixed by the clamp-iron cross I to the top of the stone. When
the stone is new, its thickness is about 14 inches, and it is made to
answer for grinding till it be reduced to about 8 inches, by lowering
the clamp I upon the shaft, so that it may continue to keep its hold of
the stone. The manner in which the grindstones are turned, is obvious
from inspection of _fig._ 901., where the horizontal axis L, which
receives its impulsion from the great water-wheel, turns the prolonged
shaft L´, or leaves it at rest, according as the clutch _l_, _l´_, is
locked or opened. This second shaft bears the three bevel wheels M, M,
M. These work in three corresponding bevel wheels M´ M´ M´, made fast
respectively to the three vertical shafts of the millstones, which pass
through the cast-iron guide tubes M´´ M´´. These are fixed in a truly
vertical position by the collar-bar _m´´_, _m´_, _fig._ 902. In this
figure we see at _m_ how the strong cross-bar of cast iron is made fast
to the wooden beams which support all the upper mechanism of the
mill-work. The bearing _m´_ is disposed in an analogous manner; but it
is supported against two cast-iron columns, shown at L´´ L´´, in _fig._
901. The guide tubes M´´ are bored smooth for a small distance from each
of their extremities, and their interjacent calibre is wider, so that
the vertical shafts touch only at two places. It is obvious, that
whenever the shaft L´ is set a-going, it necessarily turns the wheels M
and M´, and their guide tubes M´´; but the vertical shaft may remain
either at rest, or revolve, according to the position of the lever click
or catch K, at the top, which is made to slide upon the shaft, and can
let fall a finger into a vertical groove cut in the surface of that
shaft. The clamp-fork of the click is thus made to catch upon the
horizontal bevel-wheel M´, or to release it, according as the lever K is
lowered or lifted up. Thus each millstone may be thrown out of or into
geer at pleasure.
[Illustrations: 904 905 906 907]
These stones make upon an average 11 or 12 turns in a minute,
corresponding to 3 revolutions of the water-wheel, which moves through a
space of 3 feet 4 inches in the second, its outer circumference being 66
feet. The weight of the upper stone, with its iron mountings, is about 6
cwt., when new. The charge of each mill in dry material is 2 cwt.; and
the water may be estimated at from one-half to the whole of this weight;
whence the total load may be reckoned to be at least 3 cwt.; the stone,
by displacement of the magma, loses fully 400 pounds of its weight, and
weighs therefore in reality only 2 cwt. It is charged in successive
portions, but it is discharged all at once. When the grinding of the
siliceous or felspar matters is nearly complete, a remarkable
phenomenon occurs; the substance precipitates to the bottom, and assumes
in a few seconds so strong a degree of cohesion, that it is hardly
possible to restore it again to the pasty or magma state; hence if a
millstone turns too slowly, or if it be accidentally stopped for a few
minutes, the upper stone gets so firmly cemented to the under one, that
it is difficult to separate them. It has been discovered, but without
knowing why, that a little vinegar added to the water of the magma
almost infallibly prevents that sudden stiffening of the deposit and
stoppage of the stones. If the mills come to be set fast in this way,
the shafts or geering would be certainly broken, were not some safety
provision to be made in the machinery against such accidents. Mr. Hall’s
contrivance to obviate the above danger is highly ingenious. The clutch
_l_, _l´_, _fig._ 901., is not a locking crab, fixed in the common way,
upon the shaft L; but it is composed, as shown in _figs._ 904, 905, 906,
907, of a hoop _u_, fixed upon the shaft by means of a key, of a collar
_v_, and of a flat ring or washer _x_, with four projections, which are
fitted to the collar _v_, by four bolts _y_. _Fig._ 905. represents the
collar _v_ seen in front; that is, by the face which carries the clutch
teeth; and _fig._ 906. represents its other face, which receives the
flat ring _x_, _fig._ 907., in four notches corresponding to the four
projections of the washer-ring. Since the ring _u_ is fixed upon the
shaft L, and necessarily turns with it, it has the two other pieces at
its disposal, namely the collar _v_, and the washer _x_, because they
are always connected with it by the four bolts _y_, so as to turn with
the ring _u_, when the resistance they encounter upon the shaft L´ is
not too great, and to remain at rest, letting the ring _u_ turn by
itself, when that resistance increases to a certain pitch. To give this
degree of friction, we need only interpose the leather washers _z_,
_z´_, _fig._ 904.; and now as the collar _coupling-box_, _v_, slides
pretty freely upon the ring _u_, it is obvious that by tightening more
or less the screw bolts _y_, these washers will become as it were a
lateral brake, to tighten more or less the bearing of the ring _u_, to
which they are applied: by regulating this pressure, every thing may be
easily adjusted. When the resistance becomes too great, the leather
washers, pressed upon one side by the collar _v_, of the washer _x_, and
rubbed upon the other side by the prominence of the ring _u_, get heated
to such a degree, that they are apt to become carbonized, and require
replacement.
This safety clutch may be recommended to the notice of mechanicians, as
susceptible of beneficial application in a variety of circumstances.