case-hardened pinions should not be used, in consequence of their
wearing with great unevenness, but such persons should be reminded
that this objection is much greater in the instance of cast-iron
wheels. A case came under my notice some time since of a clock made by
a London house, with iron wheels, which after comparatively little
time became entirely worn out and had to be removed, a result not at
all surprising to those who are aware of the porous nature of iron.
The TEETH OF WHEELS have to be made with the greatest skill and care
in order that the entire mechanism shall work without friction, and
shall not only temporarily keep time with regularity, but shall last
for many years without renewal. Teeth should fit into one another
without a squeezing pressure (which is equivalent to friction), but
with exact uniformity of contact, the action being almost entirely
between the teeth separating from each other and not between those
which are approaching, i.e. in technical language, the action should
be after the line of centres of the wheels and not before it.
Church clocks were accustomed formerly to be made to go for
thirty-four hours, and to be wound up every day; by the frequency of
which winding the clock could be made to keep time with great
accuracy, for regulating could be attended to as frequently, and no
great variation could well occur in twenty-four hours. But the
regulating, as a matter of course, requires a regulator, or standard,
of time, which is not always to be found in country places, nor even
is the man in charge of clock-winding always in possession of a watch
sufficiently accurate to convey the time from the regulator if there
were one to the Church clock. Of late, Church clocks are made to go
eight days, and so the labour of frequent winding has been saved,
while at the same time by extra care in the manufacture and fixing of
a clock, there need be no necessity for frequently regulating it.
PENDULUMS.
Whether the credit of practically applying the mathematical theory and
properties of the pendulum was or was not due to Huygens the Dutchman,
we have seen that Harris, a London clockmaker, put up the first
pendulum clock in St Paul's Church, Covent Garden, in 1621. The great
advance upon this discovery was that the pendulum bob must move not in
a circle but a cycloid; and that back and front should be alike both
in weight and shape to secure regular vibration. Cylindrical bobs are
now in general use for large clocks. The old iron rod pendulums were
soon discovered to be affected considerably by variations of heat and
cold,--the difference between winter and summer being ascertained to
amount to the loss of a minute a week. Harrison's gridiron pendulum
was one of the chief endeavours to prevent such variation, followed
after a long interval by other ingenious inventions, which gained
temporary approval and gradually fell into disuse. Room should be
provided by the architect of every clock-tower in the chamber below
that containing the movement, to allow of the swing of a 15-foot
pendulum.
FALL OF THE WEIGHTS.
We have seen that the position in which a clock is placed in regard to
the dial or dials whose hands it is to drive is a matter requiring
some attention. Properly the floor of the clock-chamber should be so
planned that the clock might stand immediately behind, and level with
the dials; for there is extra expense and inconvenience connected with
any more distant situation of the works,--the fall of the weights
being sometimes difficult in such case to be provided for. The weights
should hang, wherever it is possible so to arrange, immediately from
the barrel to which they are affixed, without the intervention of
pulleys of any kind, and much expense may be saved by providing for
the descent of the weights to a considerable depth below the
clock-chamber. As an instance however of the extent to which such
difficulties can be overcome, I may mention that the hands of my great
clock at the International Exhibition were situated nearly 400 feet
from the clockworks, while the weights were carried by iron wire ropes
over pulleys below the floor to a distance of 200 feet from the
movement, then over another pulley fixed at a height of 80 feet from
the ground.
The ESCAPEMENT is perhaps the most important part of a clock.
CROWN-WHEEL ESCAPEMENT.
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| [Illustration: Escapement.] |
+----------------------------------------------------------------------+
This is the earliest known escapement, and is to be found, as we have
said, in Henry de Wyck's clock, all the difference between his
escapement and the above being that one of the weights in de Wyck's
balance is now set in a vertical instead of a horizontal plane. The
bent end or fork seen in the illustration connects the pendulum with
that arm technically called the crutch.
THE ANCHOR ESCAPEMENT.
After the crown-wheel escapement, the anchor escapement, invented by
Dr Hooke or one of his contemporaries, came into general use, and
remains so still; but it is not generally applied to those clocks
which are required to go with the nicest accuracy.
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| [Illustration: Anchor Escapement.] |
+----------------------------------------------------------------------+
In the next illustration the tooth is seen escaping from the left
pallet at the moment of the right pallet's infringing upon the
opposite tooth, the pendulum is therefore to be seen still rising a
little to the left, and will thus cause the wheel to recoil a little;
upon its return the pallet and pendulum are again urged to the right,
and so the impulse is continued which is necessary to maintain the
motion.
THE DEAD-BEAT ESCAPEMENT.
invented by Graham is the one in most general use for the best clocks
made by London makers of the highest repute.
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| [Illustration: Dead-beat Escapement] |
+----------------------------------------------------------------------+
FRENCH SINGLE-PIN ESCAPEMENT.
This is a simple and ingenious escapement (see next page), which after
being used for some time in both France and England went out of use,
when, but recently, it was re-invented by a London watch-maker. The
teeth are pins of steel set in the face of the wheel, and the upper
half of each cylinder cut off as well as a small portion of the under
or acting side. This escapement has one great advantage--that if a pin
becomes worn or injured it is easily replaced, whereas in a wheel, if
one tooth is damaged the wheel itself is worthless.
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| [Illustration: French Single-Pin Escapement.] |
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THREE-LEGG'D GRAVITY ESCAPEMENT.
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| [Illustration: Another.] |
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The above illustration represents a regulator escapement as it would
appear in a front view; the pallets are lifted by the three central
pins. The locking teeth vary in size from one to nearly two inches.
The horizontal pieces projecting from the top of the pallets form the
adjustment for the arc of the pendulum.
The great advantages possessed by this escapement over all other
gravity escapements, &c., are as follows:--
1. It requires no oil.
2. The angle of the detent planes reduces the friction to almost nil.
3. As the impulse and the unlocking are in one direction, the
escapement is unlocked without recoil of impulse arms.
4. No impending force to the pendulum from inertia of impulse arms.
5. The hold in the stops can be increased or diminished to any
practical extent by reason of the inverted impulse arms.
6. Less affected by any disturbing forces of the train in proportion
to the pressure on the stops.
7. Will bear more weight and give more power to the train without
increasing the arc of oscillation.
8. No possibility of tripping under any increase of motive power.
9. The minimum arc of vibration to unlock is 8-tenths of a degree.
Other escapements of similar construction require from 4° to 7°.
10. Take less weight for the motive power in proportion to the
difference of pressure and draught on the lockings.
11. Unlocks by gravitation instead of by the pendulum and at the time
of impulse.
12. Requires no fly nor remontoir, and thus reduces the weight of the
motive power by one half.
13. The impulse giving motion to the pendulum increases as the force
of gravity on the pendulum decreases. A great advantage over those
escapements in which the unlocking is done by the pendulum when its
momentum is nearly expended and at the extremity of its arc of
vibration.
14. The angle of the detent planes can be set so as not only to offer
no resistance to the unlocking, but to give an actual impulse in the
same manner as the impulse pallets of a dead escapement. This
completely frees the impulse which gives motion to the pendulum from
any retarding influence of the train.
15. The arc of vibration is more equal in this than in any other
gravity escapement.
16. It is not so liable to stop in consequence of a diminution of arc
from the variation of motive force in train.
17. It will answer for regulators as well as for turret clocks, its
arc of vibration being from 1° to 3°.
DOUBLE THREE-LEGG'D ESCAPEMENT.
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| [Illustration: Double Three-legg'd Gravity Escapement.] |
+----------------------------------------------------------------------+
This escapement is chiefly designed for turret clocks with heavy
dial-work requiring much power on the scape-wheel. The peculiarity
consists of two locking wheels with one set of lifting pins between
them. The wheels are set so that the pallets may lie between, and the
pallets fall with the pendulum clear of all other contact. The pallet
D for instance has its stop in front for the wheel A B C to act upon,
and the E stop is acted upon only by _a b c_, the E and A being on
different planes. In this escapement, by making the teeth longer and
the pallets shorter, the resistance of the pendulum is much reduced,
and the stride of the pallets being wider, the actual weight required
of them is considerably lessened,--a point of some importance.
THE REMONTOIRE.
is an invention which, being derived from the French, still bears its
French title, and consists of either a train remontoire, or a gravity
or remontoire escapement, in which latter the impulse is not given to
the pendulum directly by the clock-train or weight, but by some small
weight lifted up or a small spring bent up by the clock-train at every
beat of the pendulum, so as to secure a uniform and constant impulse,
the remontoire weights being lifted either faster or slower according
to need. The train remontoire differs from the escapement but
slightly, the chief difference being that the small weight or spring
which gives the impulse to the pendulum is not wound up at every
beat, but at some larger interval, seldom more than half a minute. Its
effect is to counteract the various errors to which large clocks
driving heavy hands are always liable, and to diminish the friction
which arises from the use of heavy weights--these being in very large
clocks almost incredibly heavy; for instance, the weights used by me
for my clock in the Great International Exhibition of 1862 amounted to
more than two tons. Whatever the cause of inequality of movement in
the clock, whether it be dust or dirt, or insufficient oil, or whether
it be wind delaying or expediting the progress of the hands on the
dial, the remontoire regulates and counteracts.
THE DIALS.
The utility of a Public Clock is considerably enhanced by its being
provided with a dial marking the time in the simplest and most
unmistakeable lines, so that it may readily be ascertained at any
reasonable distance from the clock-tower what is the hour either by
day or night. In order that this important requisite may be attained,
it is of course necessary that the dial shall be so constructed as to
be visible both by night and day, and so arises the necessity for
providing illuminating power either from within or from without. Now
the simplest method, and perhaps also in the end the least
objectionable, is that followed at the Horse Guards, where the dial
forms part of the tower itself, and is lighted not from within, but
from without. The advantage of this arrangement is, that the architect
can make the dial harmonize with the character of the building, that
the illuminating power is kept apart from the clock, and if the centre
of the dial be slightly sunk the hands may be brought quite close to
the face, so as to prevent any seeming error in time, as is sometimes
caused by the convexity of a copper dial. The figures too, having been
once carefully divided and cut into the stone, are renewed, so to
speak, by merely being painted over.
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| [Illustration: Memorial Turret Clock Dial.] |
+----------------------------------------------------------------------+
Dials may be made of any material, wood, stone, slate, iron, brass,
copper, and coloured or semi-opaque glass. Copper dials possess many
advantages, and these have been of late years preferred, except where
more ornamental dials are required, in which case slate and skeleton
frames are used with good effect. The large dial of my great clock
which was placed over the principal entrance of the International
Exhibition Building in the Cromwell Road was of slate, elaborately
enamelled with white and gold on a blue ground. Another kind of dial
having a good effect is that erected by myself some time since for Sir
Moses Montefiore, at the Synagogue, Ramsgate, consisting of a skeleton
or framework of iron fitted with Minton or encaustic tiles. A dial
such as this can thus be made with comparatively little expense during
the erection of the tower, and the architect can then, as I have said,
design it so as to be in keeping with the edifice; the Minton tiles
have also the advantage of being almost indestructible, and of being
made of any pattern or colour. The chief points to remember are that
the dials should be slightly sunk in the centre so as to allow the
hour hand to traverse in the sinking point close to the disc and the
figures, and especially that the dial should be made large enough to
distinctly show the hour. Properly the dial should never be less in
diameter than one-tenth of the number of feet which it is distant from
the ground, and in all cases where it is possible I should recommend
it to be much larger than this. The dials of St Paul's and Westminster
are larger than they would be under the above rule, and they are
certainly not too large. As to the colour of the dials, figures, and
hands, there is not much choice; dark ground and gilt figures, or
white ground with black figures, or a skeleton frame with gilt figures
are the chief in use. In the white semi-transparent dials with opaque
figures used for illuminated clocks, the time, which is seen with
sufficient distinctness by night when the light is behind the figures,
is not as clearly indicated by day. To remedy this defect an invention
has been applied by which the dial when illuminated at night throws
out a beautiful transparent light admirably marking the position of
the figures and hands, which being black or dark blue, or even
strongly gilt, can also be distinctly seen by day, even as clearly as
the long-approved copper dials painted black with gilt figures.
THE HANDS
should be most carefully made, and like the figures should be painted
of a colour which shall most powerfully contrast with that of the
dial. The hands are almost invariably made of copper strengthened by
diaphragms, and poised from the inside. In some old-fashioned clocks
in which the hands have been poised from the outside the effect has
been produced of a third hand, and numerous mistakes caused thereby.
As to the shape of the hands, there is but one simple rule, namely,
that the less of ornamentation in them the better. The minute hand
should be perfectly plain, with a tapering but not too fine point,
extending to the top of the figures; the hour hand should be of equal
breadth and plainness, but its point should be more marked by perhaps
an arrowhead or heart-shaped tip only reaching to the bottom of the
figures. With large hands counterpoises are found necessary, and these
should be placed inside the dial if possible, for they are when
outside sometimes mistaken for the point of the hour hand. If a
counterpoise must be placed outside, it is better to arrange that it
shall be as little as possible, and that the inside counterpoise make
up the difference, giving to the latter perhaps two thirds, and one
third to the former,--but in any case care has to be taken to prevent
the counterpoise appearing like a hand.
THE FRAME.
The old-fashioned clock-frame, known in the trade as the 'bedstead,'
is now generally superseded by the horizontal frame originally
introduced by the French, which possesses the special advantage of
not only being durable and strong, but that it allows of any part of
the clock which may have been injured, or may require cleaning, being
easily taken out and replaced without interfering with other portions
of the mechanism,--any wheel can be separately handled and removed. In
the old upright frame which is even now still in use by some of the
more ancient firms of clockmakers, if any part of the clock be injured
the entire machine must be taken to pieces.
THE FIXING
of a Turret Clock requires much careful forethought and experienced
labour; because whatever oversight has been made by the architect in
planning the clock-room must be made good by the clockmaker who has to
fix up a public time-piece. In the first place the latter will take
care that the supports of the clock shall be sufficiently strong and
free from vibration, and that the movement shall be bolted securely to
the iron girders, or strong oak beams provided for the purpose; he
will remember that when it is intended that the clock shall strike the
hours and quarters, that the bell or bells should be hung as high in
the tower as possible, so that when the stroke of the hammer is given
by a perfect fall of the weights, the louvres of the tower should be
so arranged as to bring out the full sound of the bell, as in the
case of the bell at St Paul's cathedral, which, though only weighing 5
tons 4 cwts., is frequently heard on clear nights as far as Windsor.
He will in a word require to be acquainted with all the points of
importance attached to his rather intricate duty, or he may by failure
render nugatory the best workmanship that could be bestowed in
clockmaking. The wiser arrangement as to clock-fixing is to intrust
the duty to the clockmaker, and he will then necessarily bear the sole
responsibility of any mistake.
THE WINDING and keeping in order is, as we have said, a less laborious
task as respects modern clocks than those which were made fifty years
ago, inasmuch as, although it is the duty of a clock-winder to watch
daily the action of the time-piece under his charge, he need not
perform his winding duties oftener than once a week. He must be on the
alert to observe any effect produced by the action of the wind or the
fall of snow upon the hands of the clock, which under certain
conditions is not uncommon; he must note by some good regulator any
tendency to variation in the Church clock, and he must also observe
the Equation of Time, which is the difference between true and mean
solar time for each day, and which is not quite the same for every
year, because it moves on about a quarter of a day in each year until
leap year comes and puts it back again. The Equation may be reckoned
by an Equation Table, or by the time mentioned in the Almanacs as
'clock before' or 'clock after sun.' It is obviously a very important
requisite for good time-keeping that good horological instruments
shall be intrusted to skilful and careful hands. In many instances it
has happened that escapements made upon the truest scientific
principles, and set going in thorough working order, have been so
injured by the mechanical genius of the village (some blundering
sexton, or some jack-of-all-trades, whose education in mechanism must
be exercised at the parish expense), that the new clock with all its
merits has been seriously damaged. In such a case the clockmaker had
better be at once consulted.
A MODERN TURRET CLOCK DESCRIBED.
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| [Illustration: A Modern Turret Clock.] |
| |
| [Illustration: Hour-Wheel and Snail.] |
+----------------------------------------------------------------------+
The Turret Clock which the highest skill and the best experience of
the value of the latest improvements can produce, may be thus
described:--