Chapter VIII: Part 8

To abolish errors arising from the changes in the force driving the
escapement, what is known as the "remontoire" system was adopted. It
first came into use for watches, which was perhaps natural, seeing
that the driving force of a watch is not a uniform weight like that of
a clock, but depends on springs, which are far less trustworthy. The
idea of a remontoire is to disconnect the escapement from the clock
train, and to give the escapement a driving power of its own, acting
as directly as possible on the pallets without the intervention of a
clock-train containing many wheels. The escapement is thus as it were
made into a separate clock, which of course needs repeated winding,
and this winding is effected by the clock-train. From this it results
that variations in the force transmitted by the clock-train merely
affect the speed at which the "rewinding" of the escapement is
effected, but do not affect the force exerted by the driving power of
the escapement.

Train remontoires.

There are several modes of carrying out this plan. The first of them
is simply to provide the scape-wheel with a weight or spring of its
own, which spring is wound up by the clock-train as often as it runs
down. Contrivances of this kind are called train remontoires. In
arranging such a remontoire it is obvious that the clock-train must be
provided with a stop to prevent it from overwinding the scape-wheel
weight or spring, and further, that there must be on the scape-wheel
some sort of stud or other contrivance to release the clock-train as
soon as the scape-wheel weight or spring has run down and needs
rewinding. We believe the first maker of a large clock with a train
remontoire was Thomas Reid of Edinburgh, who described his apparatus
in his book on _Horology_ (1819). The scape-wheel was driven by a
small weight hung by a Huygens's endless chain, of which one of the
pulleys was fixed to the arbor, and the other rode upon the arbor,
with the pinion attached to it, and the pinion was driven and the
weight wound up by the wheel below (which we will call the third
wheel), as follows. Assuming the scape-wheel to turn in a minute, its
arbor has a notch cut half through it on opposite sides in two places
near to each other; on the arbor of the wheel, which turns in ten
minutes, suppose, there is another wheel with 20 spikes sticking out
of its rim, but alternately in two different planes, so that one set
of spikes can only pass through one of the notches in the scape-wheel
arbor, and the other set only through the other. Whenever, then, the
scape-wheel completes a half-turn, one spike is let go, and the third
wheel is able to move, and with it the whole clock-train and the
hands, until the next spike of the other set is stopped by the
scape-wheel arbor; at the same time the pinion on that arbor is turned
half round, winding up the remontoire weight, but without taking its
pressure off the scape-wheel. Reid says that, so long as this
apparatus was kept in good order, the clock went better than it did
after it was removed in consequence of its getting out of order from
the constant banging of the spikes against the arbor.

A clock at the Royal Exchange, London, was made in 1844 on the same
principle, except that, instead of the endless chain, an internal
wheel was used, with the spikes set on it externally, which is one of
the modes by which an occasional secondary motion may be given to a
wheel without disturbing its primary and regular motion. The following
is a more simple arrangement of a gravity train remontoire, much more
frequently used in principle. Let E in fig. 15 be the scape-wheel
turning in a minute, and e its pinion, which is driven by the wheel D
having a pinion d driven by the wheel C, which we may suppose to turn
in an hour. The arbors of the scape-wheel and hour-wheel are distinct,
their pivots meeting in a bush fixed somewhere between the wheels. The
pivots of the wheel D are set in the frame AP, which rides on the
arbors of the hour-wheel and scape-wheel, or on another short arbor
between them. The hour-wheel also drives another wheel G, which again
drives the pinion f on the arbor which carries the two arms fA, fB;
and on the same arbor is set a fly with a ratchet, like a common
striking fly, and the numbers of the teeth are so arranged that the
fly will turn once for each turn of the scape-wheel. The ends of the
remontoire arms fA, fB are capable of alternately passing the notches
cut half through the arbor of the scape-wheel, as those notches
successively come into the proper position at the end of every
half-minute; as soon as that happens the hour-wheel raises the movable
wheel D and its frame through a small angle; but, nevertheless, that
wheel keeps pressing on the scape-wheel as if it were not moving, the
point of contact of the wheel C and the pinion d being the fulcrum or
centre of motion of the lever AdP. It will be observed that the
remontoire arms fA, fB have springs set on them to diminish the blow
on the scape-wheel arbor, as it is desirable not to have the fly so
large as to make the motion of the train, and consequently of the
hands, too slow, to be distinct.

Another kind of remontoire is on the principle of one bevelled wheel
lying between two others at right angles to it. The first of the
bevelled wheels is driven by the train, and the third is fixed to the
arbor of the scape-wheel; and the intermediate bevelled wheel, of any
size, rides on its arbor at right angles to the other two arbors which
are in the same line. The scape-wheel will evidently turn with the
same average velocity as the first bevelled wheel, though the
intermediate one may move up and down at intervals. The transverse
arbor which carries it is let off and lifted a little at half-minute
intervals, as in the remontoire just now described; and it gradually
works down as the scape-wheel turns under its pressure, until it is
freed again and lifted by the clock-train.

In all these gravity remontoires, however, only the friction of the
heavy parts of the train and the dial-work is got rid of, and the
scape-wheel is still subject to the friction of the remontoire wheels,
which, though much less than the other, is still something
considerable. Accordingly, attempts have frequently been made to drive
the scape-wheel by a spiral spring, like the mainspring of a watch.
One of these was described in the 7th edition of this encyclopaedia;
and Sir G. Airy invented another on the same principle, of which one
specimen is still going well. One of the best forms of such a
remontoire is shown in fig. 16, in which A, B, D, E, e, f are the same
things as in fig. 15. But e, the scape-wheel pinion, is no longer
fixed to the arbor, nor does it ride on the arbor, as had been the
case in all the previous spring remontoires, thereby producing
probably more friction than was saved in other respects; but it rides
on a stud k, which is set in the clock frame. On the face of the
pinion is a plate, of which the only use is to carry a pin h (and
consequently its shape is immaterial), and in front of the plate is
set a bush b, with a hole through it, of which half is occupied by the
end of the stud k, to which the bush is fixed by a small pin, and the
other half is the pivot-hole for the scape-wheel arbor. On the arbor
is set the remontoire spring s (a moderate-sized musical-box spring is
generally used), of which the outer end is bent into a loop to take
hold of the pin h. In fact, there are two pins at h, one a little
behind the other, to keep the coils of the spring from touching each
other. Now, it is evident that the spring may be wound up half or a
quarter of a turn at the proper intervals without taking the force off
the scape-wheel, and also without affecting it by any friction
whatever. When the scape-wheel turns in a minute, the letting-off
would be done as before described, by a couple of notches in the
scape-wheel arbor, through which the spikes A, B, as in fig. 15, would
pass alternately. During the half-minute that the spring is running
down the impulse on the pendulum constantly diminishes; but this error
is small if the spring be properly shaped, and besides, being
periodic, does not affect the _average_ time-keeping of the clock. It
would be inadmissible in astronomical clocks where each particular
second has always to be true. In clocks with only three wheels in the
train it is best to make the scape-wheel turn in two minutes. In that
case four notches and four remontoire arms are required, and the fly
makes only a quarter of a turn. Lord Grimthorpe made the following
provision for diminishing the friction of the letting-off work. The
fly pinion f has only half the number of teeth of the scape-wheel
pinion, being a lantern pinion of 7 or 8, while the other is a leaved
pinion of 14 or 16, and therefore the same wheel D will properly drive
both, as will be seen hereafter. The scape-wheel arbor ends in a
cylinder about 5/8 in. in diameter, with two notches at right angles
cut in its face, one of them narrow and deep, and the other broad and
shallow, so that a long and thin pin B can pass only through one, and
a broad and short pin A through the other. Consequently, at each
quarter of a turn of the scape-wheel, the remontoire fly, on which the
pins A, B are set on springs, as in fig. 15, can turn half round. It
is set on its arbor f by a square ratchet and click, which enables the
spring to be adjusted to the requisite tension to obtain the proper
vibration of the pendulum. A better construction, afterwards
introduced, is to make the fly separate from the letting-off arms,
whereby the blow on the cylinder is diminished, the fly being allowed
to go on as in the gravity escapement. It should be observed, however,
that even a spring remontoire requires a larger weight than the same
clock without one; but as none of that additional force reaches the
pendulum, that is of no consequence. The variation of force of the
remontoire spring from temperature, as it only affects the pendulum
through the medium of the dead escapement, is far too small to produce
any appreciable effect; and it is found that clocks of this kind, with
a compensated pendulum 8 ft. long, and weighing about 2 cwt., will not
vary above a second a month, if the pallets are kept clean and well
oiled. No turret clock without either a train remontoire or a gravity
escapement will approach that degree of accuracy.

The introduction of this remontoire led to another very important
alteration in the construction of large clocks. Hitherto it had always
been considered necessary, with a view to diminish the friction as far
as possible, to make the wheels of brass or gun-metal, with the teeth
cut in an engine. The French clockmakers had begun to use cast iron
striking parts, and cast iron wheels had been occasionally used in the
going part of inferior clocks for the sake of cheapness; but they had
never been used in any clock making pretensions to accuracy. But in
consequence of the success of a clock shown in the 1851 Exhibition, it
was determined by Sir G. Airy and Lord Grimthorpe (then E. Denison),
who were jointly consulted by the Board of Works about the great
Westminster clock in 1852, to alter the original requisition for
gun-metal wheels there to cast iron. But cast iron wheels must drive
cast iron pinions, for they will wear out steel.

Gravity escapements.

The next kind of remontoire still leaves the scape-wheel linked up
with the clock-train, but makes it wind up the pallets which are held
raised up till their action is wanted, when they are allowed to drop
gently on the crutch or the pendulum rod. In this case the two arms of
the anchor are usually divided and mounted on separate shafts so as to
act independently. This idea was first started by Thomas Mudge
(1717-1794) and Alexander Cumming (1733-1814). Mudge's escapement is
shown in fig. 17. The tooth A of the scape-wheel is resting against
the stop or detent a at the end of the pallet CA, from the axis or
arbor of which descends the half-fork CP to touch the pendulum. From
the other pallet CB descends the other half-fork CO. The two arbors
are set as near the point of suspension, or top of the pendulum
spring, as possible. The pendulum, as here represented, must be moving
to the right, and just leaving contact with the left pallet and going
to take up the right one; as soon as it has raised that pallet a
little it will evidently unlock the wheel and let it turn, and then
the tooth B will raise the left pallet until it is caught by the stop
b on that pallet, and then it will stay until the pendulum returns and
releases it by raising that pallet still higher. Each pallet therefore
descends with the pendulum to a lower point than that where it is
taken up, and the difference between them is supplied by the lifting
of each pallet by the clock, which does not act on the pendulum at
all; so that the pendulum is independent of all variations of force
and friction in the train. This escapement is said by Lord Grimthorpe,
in his _Rudimentary Treatise on Clocks_, first published in 1850, to
be liable to trip, the pallets being apt to be jerked by the pendulum,
so that the teeth slip past the hook, and the wheel flies round. This,
however, appears entirely a matter of construction. The really weak
point is that while the impulses on the pendulum due to the
gravitational fall of the arms are uniform, the force which has to be
exercised by the pendulum in unlocking them from the scape-wheel
varies with the pressure of the clock-train. Hence we miss the
compensation which is so beautiful a result of Graham's escapement. To
avoid this, J. M. Bloxam, a barrister, proposed about the middle of
the 19th century his legged gravity escapement (fig. 18). By this
arrangement the parts of the scape-wheel which lifted the gravity
arms were brought as near to the axis of the scape-wheel as possible,
while the locking arms were brought as far from the axis as possible
so that the pressure should be light. The pallet arbors were cranked,
to embrace the pendulum-spring, so that their centres of motion might
coincide with that of the pendulum as nearly as possible--perhaps an
unnecessary refinement; at least the three-legged and four-legged
gravity escapements answer very well with the pallet arbors set on
each side of the top of the spring. The size of the wheel determines
the length of the pallets, as they must be at such an angle to each
other that the radii of the wheel when in contact with each stop may
be at right angles to the pallet arm; and therefore, for a wheel of
this size, the depth of locking can only be very small. The pinion in
Bloxam's clock only raises the pallet through 40' at each beat; i.e.
the angle which we call [gamma], viz. the amplitude of the pendulum
when it begins to lift the pallet, is only 20'; and probably, if it
were increased to anything like a/sqrt(2), where a is the semiarc of
swing, the escapement would trip immediately. The two broad pins
marked E, F, are the fork-pins, and A and B are the stops. The clock
which Bloxam had went very well; but it had an extremely fine train,
with pinions of 18; and nobody else appears to have been able to make
one to answer.

Bloxam's escapement was modified in form by Lord Grimthorpe, his chief
improvement being the addition of a fly vane, which, however, had
previously been used for remontoires to steady the motion. He tried
various modifications of construction, but finally adopted the
"four-legged" and "double-three-legged" forms as being the most
satisfactory, the former for regulators and the latter for large
clocks. Fig. 19 is a back view of the escapement part of an
astronomical clock with the four-legged wheel; seen from the front the
wheel would turn the other way. The long locking teeth are made about
2 in. long from the centre, and the lifting pins, of which four point
forwards while four other intermediate ones point backwards, are at
not more than 1/30 of the distance between the centres EC, of the
scape-wheel and pallets; or rather C is the top of the pendulum spring
to which the pallets Cs, Cs' converge, though the resultant of their
action is a little below C. It is not worth while to crank them as
Bloxam did, in order to make them coincide exactly with the top of the
pendulum, as the friction of the beat pins on the pendulum is
insignificant, and even then would not be quite destroyed. The pallets
are not in the same plane, but one is behind and the other in front of
the wheel, with one stop pointing backwards and the other forwards to
receive the teeth alternately--it does not matter which; in this
figure the stop s is behind and the stop s' forward. The pendulum is
now going to the right, and just beginning to lift the right pallet
and free the stop s'; then the wheel will begin to turn and lift the
other pallet by one of the pins which is now lowest, and which moves
through 45 deg. across the line of centres, and therefore lifts with
very little friction. It goes on till the tooth now below s reaches s
and is stopped there. Meanwhile the pallet Cs' goes on with the
pendulum as far as it may go, to the end of the arc which we have
called [alpha], starting from [gamma]; but it falls with the pendulum
again, not only to [gamma] but to -[gamma] on the other side of 0, so
that the impulse is due to the weight of each pallet alternately
falling through 2[gamma]; and the magnitude of the impulse also
depends on the obliqueness of the pallet on the whole, i.e. on the
distance of its centre of gravity from the vertical through C. The fly
KK' is set on with a friction spring like the common striking-part
fly, and should be as long as there is room for, length being much
more effective than width.

The double three-legged gravity escapement, which was first used in
the Westminster clock, is shown in fig. 20. The principle of it is the
same as of the four-legs; but instead of the pallets being one behind
and the other in front of the wheel, with two sets of lifting pins,
there are two wheels ABC, abc, with the three lifting pins and the two
pallets between them like a lantern pinion. One stop B points forward
and the other A backward. The two wheels have their teeth set
intermediately or 60 deg. apart, though that is not essential, and the
angle of 120 deg. may be divided between them in any other
proportions, as 70 deg. and 50 deg., and in that way the pallets may
be still more oblique than 30 deg. from the vertical, which, however,
is found enough to prevent tripping even if the fly gets loose, which
is more likely to happen from carelessness in large clocks than in
astronomical ones.

Of course the fly for those escapements in large clocks, with weights
heavy enough to drive the hands in all weather, must be much larger
than in small ones. For average church clocks with 1-1/4 sec. pendulum
the legs of the scape-wheels are generally made 4 in. long and the fly
from 6 to 7 in. long in each vane by 1-1/4 or 1-1/2 wide. For 1-1/2 sec.
pendulums the scape-wheels are generally made 4-1/2 radius. At
Westminster they are 6 in.

Lord Grimthorpe considered that these escapements act better,
especially in regulators, if the pallets do not fall quite on the
lifting pins, but on a banking, or stop at any convenient place, so as
to leave the wheel free at the moment of starting; just as the
striking of a common house clock will sometimes fail to start unless
the wheel with the pins has a little run before a pin begins to lift
the hammer. The best way to manage the banking is to make the
beat-pins long enough to reach a little way behind the pendulum, and
let the banking be a thin plate of any metal screwed adjustably to the
back of the case. This plate cannot well be shown in the drawings
together with the pendulum, which, it may be added, should take up one
pallet just when it leaves the other.

Chronometer spring remontoire.

In chronometer spring remontoires the pendulum, as it goes by, flips a
delicate spring and releases a small weight or spring which has been
wound up in readiness by the action of the scape-wheel and which by
leaping on to the pendulum gives it a push. One on this principle made
about the middle of the 19th century by Robert Houdin is to be seen at
the Conservatoire des Arts et Metiers. It is very complicated. The
following is more simple. In fig. 21 a scape-wheel AB has 30 pins and
360 teeth. It is engaged with a fly vane EP mounted on a pinion of 12
teeth. Each pin as it passes raises an impulse arm CD which is hooked
upon a detent K. A pall NM then engages the fly vane and prevents the
scape-wheel from moving farther. The impulse arm being now set, as the
plate F attached to the lower end of the pendulum flies past from left
to right a pall G knocks aside the detent K, and allows a pin O
projecting from the end of the impulse arm to fall upon an inclined
pallet h, which is thus urged forward. As soon as the pallet has left
the pin, the impulse arm in its further fall strikes N, which
disengages the pall at P and allows the scape-wheel to move on and
again wind up the impulse arm CD, which is then again locked by the
detent K. On the return journey of the pendulum the light pall G,
which acts the part of a chronometer spring, flips over the detent.
The pallet is double sided, h and h', so that if by chance the clock
runs down while the pendulum swings from left to right the impulse arm
will be simply raised and not smashed. It has a flat apex, on which
the pin falls before descending. The impulse given depends on the
weight of the impulse arm and may be varied at pleasure. The work done
in unlocking the detent is invariable, as it depends on the pressure
of the fly vane at P and is independent of the clock-train. The
duration of the impulse is very short--only about 1/10 of the arc of
swing. It is given exactly at the centre of the swing, and when not
under impulse the pendulum is detached.

_Clock Wheels._--Since, as we have seen, any increase in the arc of a pendulum is accompanied by a change in its going rate, it is very desirable to keep the force which acts on the pendulum uniform. This in fact is the great object of the best escapements. Inasmuch as the impulse on the pendulum, derived from the work done by a falling weight or an unwinding spring, is transmitted through a train of wheels, it is desirable that that transmission should be as free from friction and as regular as possible. This involves care in the shaping of the teeth. The object to be aimed at is that as the wheel turns round the ratio of the power of the driver to that of the driven wheel ("runner" or "follower") should never vary. That is to say, whether the back part of the tooth of the driver is acting on the tip of the tooth of the follower, or the tip of the driver is acting on the back part of the tooth of the follower, the leverage ratio shall always be uniform. For simplicity of manufacture the pinion wheels are always constructed with radial leaves, so that the surface of each tooth is a plane passing through the axis of the wheel. The semicircular rounding of the end of the tooth is merely ornamental. The question therefore is, suppose that it is desired by means of a tooth on a wheel to push a plane round an axis, what is the shape that must be given to that tooth in order that the leverage ratio may remain unaltered?

Epicycloidal teeth.

If a curved surface, known as a "cam," press upon a plane one, both being hinged or centred upon pivots A and B respectively (fig. 22), then the line of action and reaction at D, the point where they touch, will be perpendicular to their surfaces at the point of contact--that is perpendicular to BD, and the ratio of leverage will obviously be AE:BD, or AC:CB. Hence to cause the leverage ratio of the cam to the plane always to remain unaltered, the cam must be so shaped that in any position the ratio AC:CB will remain unchanged. In other words the shape of the cam must be such that, as it moves and pushes BD before it, the normal at the point of contact must always pass through the fixed point C.

If a circle PMB roll upon another circle SPT (fig. 23) any point M on it will generate an epicycloid MN. The radius of curvature of the curve at M will always be MP, for the part at M is being produced by rotation round the point P. It follows that a line from B to M will always be tangential to the epicycloid. If the epicycloid be a cam moving as a centre round the centre R (not shown in the figure) of the circle SPT, the leverage it will exert upon a plane surface BM moving round a parallel axis at B, will always be as BP to PR, that is, a constant; whence MN is the proper shape of a tooth to act on a pinion with radial arms and centred at B. In designing a pair of wheels to transmit motion, which is to be multiplied say 6 times in the transmission (about the usual ratio for clock wheels), if we take two circles (called the "pitch circles") touching one another with radii as 1:6, then the circumference of the smaller will roll 6 times round that of the larger. The smaller wheel will have a number of teeth, say 8 to 16, each of them being sectors of the circle (fig. 24). If there are 16 teeth, then on the surface of the driving wheel there will be 96 teeth. Each of these teeth will be shaped as the curve of an epicycloid formed by the rolling on the big circle of a circle whose diameter is the radius of the pitch circle of the pinion. Points of the teeth so formed are cut off, so as to allow of the pinion having a solid core to support it, and gaps are made into the pitch circle to admit the rounded ends of the leaves of the pinon wheel. Thus a cog-wheel is shaped out.

Clock wheels are made of hard hammered brass cut out by a wheel cutting machine. This machine consists of a vertical spindle on the top of which the wheel to be cut is fixed on a firmly resisting plate of metal of slightly smaller diameter, so as to allow the wheel to overlap. A cutter with the edges most delicately ground to the exact shape of the gap between two teeth is caused to rotate 3000 - 4000 times a minute, and brought down upon the edge of the wheel. The shavings that come off are like fine dust, but the cutter is pushed on so as to plunge right through the rim of the wheel in a direction parallel to the axis. In this way one gap is cut. The vertical spindle is now rotated one division, by means of a dividing plate, and another tooth is cut, and so the operation goes on round the wheel.

It is not desirable in clocks that the pinion wheels which are driven should have too few teeth, for this throws all the work on a pair of surfaces before the centres and is apt to produce a grinding motion. Theoretically the more leaves a pinion has the better. Pinions can be made with leaves of thin steel watch-spring. In this case quite small pinions can have 20 leaves or more. The teeth in the driving wheels then become mere notches for which great accuracy of shape is not necessary. Such wheels are easy to make and run well. Lantern pinions are also excellent and are much used in American clocks. They are easy to make in an ordinary lathe. The cog-wheels must, however, be specially shaped to fit them. They consist of a number of round pins arranged in a circle round the axis of the wheel and parallel to it. The ends are secured in flanges like the wires of a squirrel cage. The teeth of cog-wheels engage them and thus drive the wheel round. They were much used at one time but are now falling out of favour again.

Involute teeth.

It is possible to make toothed wheels that drive with perfect uniformity by using for the curve of the teeth involutes of circles. These involutes are traced out by a point on a string that is gradually unwound from a circle. They are in fact epicycloids traced by a rolling circle of infinite radius, i.e. a straight line. Involute teeth have the advantage that they roll on one another instead of sliding. When badly made they put considerable strain on the axes or shafts that carry them. Hence they have not been regarded with great favour by clockmakers.

Pitch.

By the pitch of a wheel is meant the number of teeth to the inch of circumference or diameter of the wheel; the former is called the circumferential pitch, the latter the diametral pitch. Thus if we say that a wheel has 40 diametral pitch we mean that it has 40 teeth to each inch of diameter. The circumferential pitch is of course got by dividing the diametral pitch by [pi]. Wheel-cutters are made for all sizes of pitches. If it were needed to make a pair of wheels the ratio of whose motion was say 6:1 and we determined to use a diametral pitch of 30 to the inch, that is teeth about 1/10 in. wide at the base, and if the smaller circle were to have 20 teeth, we should need a blank of a diameter of 20/30 + 2/30 = 22/30 in. for the smaller wheel, and one of 120/30 + 2/30 = 122/30 in. for the larger wheel which would have 120 teeth to the inch and be 4.06 in diameter to the tips of the teeth. The smaller toothed wheel would be .73 of an inch in diameter over all. The pitch circles of the wheels would be 2/3 and 4 in. respectively. For fine wheel work, where the driver is always much larger than the driven wheel, the epicycloidal tooth appears preferable, as it is generally considered to put less side strain on the pinion wheel. But the relative merits of the two systems have never been properly tested for clock work.

_Going Barrels._--A clock which is capable of going accurately must have some contrivance to keep it going while it is being wound up. In the old-fashioned house clocks, which were wound up by merely pulling one of the strings, and in which one such winding served for both the going and striking parts, this was done by what is called the endless chain of Huygens, which consists of a string or chain with the ends joined together, and passing over two pulleys on the arbors of the great wheels, with deep grooves and spikes in them, to prevent the chain from slipping. In one of the two loops or festoons which hang from the upper pulleys is a loose pulley without spikes, carrying the clock-weight, and in the other a small weight only heavy enough to keep the chain close to the upper pulleys. Now, suppose one of those pulleys to be on the arbor of the great wheel of the striking part, with a ratchet and click, and the other pulley fixed to the arbor of the great wheel of the going part; then (whenever the clock is not striking) the weight may be pulled up by pulling down that part of the string which hangs from the other side of the striking part; and yet the weight will be acting on the going part all the time. It would be just the same if the striking part and its pulley were wound up with a key, instead of the string being pulled, and also the same, if there were no striking part at all, but the second pulley were put on a blank arbor, except that in that case the weight would take twice as long to run down, supposing that the striking part generally requires the same weight x fall as the going part.

This kind of going barrel, however, is evidently not suited to the delicacy of an astronomical clock; and Harrison's going ratchet is now universally adopted in such clocks, and also in chronometers and watches for keeping the action of the train on the escapement during the winding. Fig. 25 (in which the same letters are used as in the corresponding parts of fig. 3) shows its construction. The click of the barrel-ratchet R is set upon another larger ratchet-wheel with its teeth pointing the opposite way, and its click rT is set in the clock frame. That ratchet is connected with the great wheel by a spring ss' pressing against the two pins s in the ratchet and s' in the wheel. When the weight is wound up (which is equivalent to taking it off), the click Tr prevents that ratchet from turning back or to the right; and as the spring ss' is kept by the weight in a state of tension equivalent to the weight itself it will drive the wheel to the left for a short distance, when its end s is held fast, with the same force as if that end was pulled forward by the weight; and as the great wheel has to move very little during the short time the clock is winding, the spring will keep the clock going long enough.

In the commoner kind of turret clocks a more simple apparatus is used, which goes by the name of the _bolt and shutter_, because it consists of a weighted lever with a broad end, which shuts up the winding-hole. When it is lifted a spring-bolt attached to the lever, or its arbor, runs into the teeth of one of the wheels, and the weight of the lever keeps the train going until the bolt has run itself out of gear. Clocks are not always driven by weights. When accuracy is not necessary, but portability is desirable, springs are used. The old form of spring became weaker as it was unwound and necessitated the use of a device called a fusee or spiral drum. This apparatus will be found described in the article WATCH.

_Striking Mechanism._--There are two kinds of striking work used in clocks. The older of them, the _locking-plate_ system, which is still used in most foreign clocks, and in turret clocks in England also, will not allow the striking of any hour to be either omitted or repeated, without making the next hour strike wrong; whereas in the _rack_ system, which is used in all English house clocks, the number of blows to be struck depends merely on the position of a wheel attached to the going part, and therefore the striking of any hour may be omitted or repeated without deranging the following ones. We shall only describe the second of these, which is the more usual in modern timepieces.

Fig. 26 is a front view of a common English house clock with the face taken off, showing the repeating or rack striking movement. Here, as in fig. 3, M is the hour-wheel, on the pipe of which the minute-hand is set, N the reversed hour-wheel, and n its pinion, driving the 12-hour wheel H, on whose socket is fixed what is called the snail Y, which belongs to the striking work exclusively. The hammer is raised by the eight pins in the rim of the second wheel in the striking train, in the manner which is obvious.

The hammer does not quite touch the bell, as it would jar in striking if it did, and prevent the full sound. The form of the hammer-shank at the arbor where the spring S acts upon it is such that the spring both drives the hammer against the bell when the tail T is raised, and also checks it just before it reaches the bell, the blow on the bell thus being given by the hammer having acquired momentum enough to go a little farther than its place of rest. Sometimes two springs are used, one for impelling the hammer, and the other for checking it. But nothing will check the chattering of a heavy hammer, except making it lean forward so as to act, partially at least, by its weight. The pinion of the striking-wheel generally has eight leaves, the same number as the pins; and as a clock strikes 78 blows in 12 hours, the great wheel will turn in that time if it has 78 teeth instead of 96, which the great wheel of the going part has for a centre pinion of eight. The striking-wheel drives the wheel above it once round for each blow, and that wheel drives a fourth (in which there is a single pin P), six, or any other integral number of turns, for one turn of its own, and that drives a fan-fly to moderate the velocity of the train by the resistance of the air, an expedient at least as old as De Vick's clock in 1379.

The wheel N is so adjusted that, within a few minutes of the hour, the pin in it raises the _lifting-piece_ LONF so far that that piece lifts the click C out of the teeth of the _rack_ BKRV, which immediately falls back (helped by a spring near the bottom) as far as its tail V can go by reason of the snail Y, against which it falls; and it is so arranged that the number of teeth which pass the click is proportionate to the depth of the snail; and as there is one step in the snail for each hour, and it goes round with the hour-hand, the rack always drops just as many teeth as the number of the hour to be struck. This drop makes the noise of "giving warning." But the clock is not yet ready to strike till the lifting piece has fallen again; for, as soon as the rack was let off, the tail of the _gathering pallet_ G, on the prolonged arbor of the third wheel, was enabled to pass the pin K of the rack on which it was pressing before, and the striking train began to move; but before the fourth wheel had got half round, its pin P was caught by the end of the lifting-piece, which is bent back and goes through a hole in the plate, and when raised stands in the way of the pin P, so that the train cannot go on till the lifting-piece drops, which it does exactly at the hour, by the pin N then slipping past it. Then the train is free; the striking wheel begins to lift the hammer, and the gathering pallet gathers up the rack, a tooth for each blow, until it has returned to the place at which the pallet is stopped by the pin K coming under it. In this figure the lifting-piece is prolonged to F, where there is a string hung to it, as this is the proper place for such a string when it is wanted for the purpose of learning the hour in the dark, and not (as it is generally put) on the click C; for if it is put there and the string is held a little too long, the clock will strike too many; and if the string accidentally sticks in the case, it will go on striking till it is run down--neither of which things can happen when the string is put on the lifting-piece.

The snail is sometimes set on a separate stud with the apparatus called a _star-wheel_ and _jumper_. On the left side of the frame we have placed a lever x, with the letters st below it, and si above. If it is pushed up to si, the other end will come against a pin in the rack, and prevent it from falling, and will thus make the clock silent; and this is much more simple than the old-fashioned "strike and silent" apparatus, which we shall therefore not describe, especially as it is seldom used now.

If the clock is required to strike quarters, a third "part" or train of wheels is added on the right hand of the going part; and its general construction is the same as the hour-striking part; only there are two more bells, and two hammers so placed that one is raised a little after the other. If there are more quarter-bells than two, the hammers are generally raised by a chime-barrel, which is merely a cylinder set on the arbor of the striking-wheel (in that case generally the third in the train), with short pins stuck into it in the proper places to raise the hammers in the order required for the tune of the chimes. The quarters are usually made to let off the hour, and this connexion may be made in two ways. If the chimes are different in tune for each quarter, and not merely the same tune repeated two, three and four times, the repetition movement must not be used for them, as it would throw the tunes into confusion, but the old locking-plate movement, as in turret clocks; and therefore, if we conceive the hour lifting-piece connected with the quarter locking-plate, as it is with the wheel N, in fig. 26, it is evident that the pin will discharge the hour striking part as the fourth quarter finishes.

But where the repetition movement is required for the quarters, the matter is not quite so simple. The principle of it may shortly be described thus. The quarters themselves have a rack and snail, &c., just like the hours, except that the snail is fixed on one of the hour-wheels M or N, instead of on the twelve-hour wheel, and has only four steps in it. Now suppose the quarter-rack to be so placed that when it falls for the fourth quarter (its greatest drop), it falls against the hour lifting-piece somewhere between O and N, so as to raise it and the click C. Then the pin Q will be caught by the click Qq, and so the lifting-piece will remain up until all the teeth of the quarter-rack are gathered up; and as that is done, it may be made to disengage the click Qq, and so complete the letting off the hour striking part. This click Qq has no existence except where there are quarters.

The method in which an alarum is struck may be understood by reference to either of the recoil escapements (figs. 1 and 7). If a short hammer instead of a long pendulum be attached to the axis of the pallets, and the wheel be driven with sufficient force, it will evidently swing the hammer rapidly backwards and forwards; and the position and length of the hammer-head may be so adjusted as to strike a bell inside, first on one side and then on the other. As to the mode of letting off the alarum at the time required: if it was always to be let off at the same time all that would be necessary would be to set a pin in the twelve-hour wheel at the proper place to raise the lifting-piece which lets off the alarum at that time. But as the time must be capable of alteration, this discharging pin must be set in another wheel (without teeth), which rides with a friction-spring on the socket of the twelve-hour wheel, with a small movable dial attached to it, having figures so arranged with reference to the pin that whatever figure is made to come to a small pointer set as a tail to the hour hand, the alarum shall be let off at that hour.

The _watchman's_ or _tell-tale_ clock, used when it is desired to make sure of a watchman being on the spot and awake all the night, is a clock with a set of spikes, generally 48 or 96, sticking out all round the dial, and a handle somewhere in the case, by pulling which one of the spikes which is opposite to it, or to some lever connected with it is pressed in. This wheel of spikes is carried round with the hour-hand, which in these clocks is generally a twenty-four hour one. It is evident that every spike which is seen still sticking out in the morning indicates that at the particular time to which that spike belongs the watchman was not there to push it in--or at any rate, that he did not. At some other part of their circuit, the inner ends of the pins are carried over a roller or an inclined plane which pushes them out again ready for business the next night. The time at which workmen arrive at their work may be recorded by providing each of them with a numbered key with which he stamps his number on a moving tape, on which also the time is marked by a clock.

_Church and Turret Clocks._--Seeing that a clock--at least the going part of it--is a machine in which the only work to be done is the overcoming of its own friction and the resistance of the air, it is evident that when the friction and resistance are much increased it may become necessary to resort to expedients for neutralizing their effects, which are not required in a smaller machine with less friction. In a turret clock the friction is enormously increased by the great weight of all the parts; and the resistance of the wind, and sometimes snow, to the motion of the hands, further aggravates the difficulty of maintaining a constant force on the pendulum; and besides that, there is the exposure of the clock to the dirt and dust which are always found in towers, and of the oil to a temperature which nearly or quite freezes it all through the usual cold of winter. This last circumstance alone will generally make the arc of the pendulum at least half a degree more in summer than in winter; and inasmuch as the time is materially affected by the force which arrives at the pendulum, as well as the friction on the pallets when it does arrive there, it is evidently impossible for any turret clock of the ordinary construction, especially with large dials, to keep any constant rate through the various changes of temperature, weather and dirt to which it is exposed. Hence special precautions, such as the use of remontoires and gravity escapements, have to be observed in the design of large clocks that have any pretensions to accuracy, in order to ensure that the arc of the pendulum is not affected by external circumstances, such as wind-pressure on the hands or dirt in the wheel-train. But such have been the improvements effected in electric clocks, that rather than go to the trouble and expense required by such precautions, it appears far preferable to keep an accurate time-piece in some sheltered position and use it with a source of electricity to drive the hands of the large dial.

_Electrical Clocks._--One of the first attempts to apply electricity to clocks was made by Alexander Bain in 1840-1850. About the same time Sir C. Wheatstone, R. L. Jones, C. Shepherd, Paul Garnier and Louis Breguet invented various forms of electrical time-keepers. It is not proposed here to go into the history of these abortive attempts. Those who desire to follow them may consult Bain, _An Account of Some Applications of the Electric Fluid to the Useful Arts_ (1843) and _Short History of Electric Clocks_ (1852); Sir Charles Wheatstone, _Trade Circular of the British Telegraph Manufactory_; C. Shepherd, _On the Application of Electro-magnetism as a Motor for Clocks_ (1851), and a list of references in the Appendix to Tobler's _Die electrischen Uhren_ (Leipzig, 1883), and a list of books given by F. Hope Jones, _Proc. Inst. Elec. Eng._, 1900, vol. 29. The history of electrical clocks is a long and complicated matter, for there are some 600 or 700 patents for these clocks in Europe and America, some containing the germs of valuable ideas but most pure rubbish. All that can be done is to select one or two prominent types of each class and give a brief description of their general construction.

It is in the apparently simple matter of making and keeping the electrical contact that most of the systems of electrical time-keeping have failed, for want of attention to the essential conditions of the problem. In practice every metal is covered with a thin film of non-conducting oxide over which is another film of moisture, oil, dirt or air. Hence what is wanted is a good vigorous push of a blunted point or edge preferably obliquely upon a more or less yielding surface so as to get a rubbing action. Thus if the stiff spring a b (fig. 28) were stabbed down on the oblique surface C D a good contact would invariably result, provided that the metals employed were gold, platinum or some not easily oxidizable metal. Or again, if a mercury surface be simply touched with a pin, the slight sparking that is produced on making the current will soon form a little pile of dirty oxide at the point of entry, and the contact will frequently fail. If it be necessary to have a mercury contact, the pin must be well driven in below the surface of the mercury or else swept through it as an oar is swept through the water. Another form of electrical contact that acts well is a knife edge brought into contact with a series of fine elastic strips of metal laid parallel to one another like the fingers of a hand. The best metal for contacts, if they are to bear hard usage, is either silver or gold or a mixture of 40% iridium with 60% of platinum. A pressure of some 15 grammes, at least, is needful to secure a good contact.

[FIG. 28.]

As to the source of current for driving electrical clocks, if Leclanche cells be used they should preferably be kept in the open air under cover so as not to dry up. If direct electric current is available from electric light mains or the accumulators used for lighting a private house, so much the better. Of course the pressure of 50 or 100 volts used for lighting would be far too great for clock-driving, where only the pressure of a few volts is required. But it is easy by the insertion of suitable resistances, as for instance one or more incandescent lamps, to weaken down the pressure of the lighting system and make it available for electric clocks, bells or other similar purposes.

Electricity is applied to clocks in three main ways:--(1) in actuating timepieces which measure their own time and must therefore be provided with pendulums or balance wheels; (2) in reproducing on one or more dials the movements of the hands of a master clock, by the aid of electric impulses sent at regular intervals, say of a minute or a half-minute; and (3) in synchronizing ordinary clocks by occasional impulses sent from some accurate regulator at a distance.