distance of the points of the long teeth from the centre of the scape wheel.
As the balance turns back, the nick V goes past the end of
the tooth b. and in consequence of its smallness it passes without
visibly affecting the motion of the scape-wheel, though of course it
does produce a very slight shake in passing. It is evident that, if it
did not pass, the tooth could not get into the nick for the next escape.
The objection to this escapement is that it requires very great
delicacy of adjustment, and the watch also requires to be worn carefully;
for, if by accident the balance is once stopped from swinging
back far enough to carry the nick V past the tooth end, it will stop
altogether, as it will lose still more of its vibration the next time from
receiving no impulse. The performance of this escapement, when
well made, and its independence of oil, are nearly equal to those of
the detached escapement; but, as lever watches are now made
sufficiently good for all but astronomical purposes, for which chronometers
are used, and they are cheaper both to make and to mend
than duplex ones, the manufacture of duplex watches has almost
disappeared.
The chronometer or detached escapement is shown at fig. 7 in the form to which it was brought by Earnshaw, and in which it has remained ever since, with the very slight difference that the pallet P, on which the impulse is given (corresponding exactly to the pallet P in the duplex escapement), is now generally set in a radial direction from the verge, whereas Earnshaw made it sloped backward, or undercut, like the scape-wheel teeth. The early history of escapements on this principle does not seem to be very clear. They appear to have originated in France; but there is no doubt that they were considerably improved by the first Arnold (John), who died in 1799. Earnshaw's watches, however, generally beat his in trials.
In fig. 7 the small tooth or cam V, on the verge of the balance,
is just on the point of unlocking the detent DT from the tooth T
of the scape-wheel; and the tooth A will immediately begin to give
Fig. 7.
the impulse on the pallet P, which, in good
chronometers, is always a jewel set in the
cylinder; the tooth V is also a jewel. This
part of the action is so evident as to require
no further notice. When the balance returns,
the tooth V has to get past the end of the
detent, without disturbing it; for, as soon as
it has been unlocked, it falls against the
banking-pin E, and is ready to receive the
next tooth B, and must stay there until it is
again unlocked. It ends, or rather begins, in a
stiffish spring, which is screwed to the block
D on the watch frame, so that it moves without
any friction of pivots, like a pendulum. The
passing is done by means of another spring VT,
called the passing spring, which can be pushed away from the body
of the detent towards the left, but cannot be pushed the other way
without carrying the detent with it. In the back vibration, therefore,
as in the duplex escapement, the balance receives no impulse,
and it has to overcome the slight resistance of the passing spring
besides; but it has no other friction, and is entirely detached from
the scape-wheel the whole time, except when receiving the impulse.
That is also the case in the lever escapement; but the impulse in that
escapement is given obliquely, and consequently with a good deal
of friction; and, besides, the scape-wheel only acts on the balance
through the intervention of the lever, which has the friction of its
own pivots and of the impulse pin. The locking-pallet T is undercut
a little for safety, and is also a jewel in the best chronometers; and
the passing spring is usually of gold. In the duplex and detached
escapements, the timing of the action of the different parts requires
great care, i.e. the adjusting them so that each may be ready to act
exactly at the right time; and it is curious that the arrangement
which would be geometrically correct, or suitable for a very slow
motion of the balance, will not do for the real motion. If the pallet P
were really set so as just to point to the tooth A in both escapements
at the moment of unlocking (as it has been drawn, because otherwise
it would look as if it could not act at all), it would run away some
distance before the tooth could catch it, because in the duplex
escapement the scape-wheel is then only moving slowly, and in the
detached it is not moving at all, and has to start from rest. The
pallet P is therefore, in fact, set a little farther back, so that it
may arrive at the tooth A just at the time when A is ready for it,
without wasting time and force in running after it. The detached
escapement has also been made on the duplex plan of having long
teeth for the locking and short ones or pins nearer the centre for the
impulse;, but the advantages do not appear to be worth the additional^
trouble, and the. force required for unlocking is not sensibly
diminished by the arrangement, as the spring D must in any case be
fairly stiff, to provide against the watch being carried in the position
in which the weight of the detent helps to unlock it.
An escapement called the lever chronometer has been several times reinvented, which implies that it has never come into general use. It is combination of the lever as to the locking and the chronometer as to the impulse. It involves a little drop and therefore waste of force as a tooth of the wheel just escapes at the "passing" beat where no impulse is given. But it should be understood that a single-beat escapement involves no more loss of force and the escape of no more teeth than a double one, except the slight drop in the duplex and this lever chronometer or others on the same principle.
There have been several contrivances for remontoire escapements; but there are defects in all of them; and there is not the same advantage to be obtained by giving the impulse to a watch-balance by means of some other spring instead of the mainspring as there is in turret-clocks, where the force of the train is liable to very much greater variations than in chronometers or small clocks.
The balance-wheel and hair-spring consist of a small wheel, usually of brass, to which is affixed a spiral, or in chronometers a helical, spring. This wheel swings through an angle of from 180° to 270° and its motions are approximately isochronous. The time of the watch can be regulated by an arm to which is attached a pair of pins which embrace the hair-spring at a point near its outer end, and by the movement of which the spring can be lengthened or shortened. The first essential in a balance-wheel is that its centre of gravity should be exactly in the axis, and that the centre of gravity of the hair-spring should also be in the axis of the balance-wheel. True isochronism is disturbed by variations in the driving force of the train or by variations in temperature, and also by variations in barometric pressure. Isochronism is produced in the first place by a proper shape of the spring and its overcoil. It is usual to time the watch's going when the mainspring is partly wound up, as well as when it is fully wound up, and then by removing parts of the hair-spring to get such an adjustment that the rate is not influenced by the lesser or greater extent to which the watch has been wound. The variations in length and still more in elasticity caused in a hair-spring by changes of temperature were for long not only a trouble to watchmakers but a bar to the progress of the art. A pendulum requires scarcely any compensation except for its own elongation by heat; but a balance requires compensation, not only for its own expansion, which increases its moment of inertia just like the pendulum, but far more on account of the decrease in the strength of the spring under increased heat. E. G. Dent, in a pamphlet on compensation balances, gave the following results of some experiments with a glass balance, which he used for the purpose on account of its less expansibility than a metal one: at 32° F., 3606 vibrations in an hour; at 66°, 3598.5; and at 100°, 3599. If therefore it had been adjusted to go right (or 3600 times in an hour) at 32°. it would have lost 712 and 812 seconds an hour, or more than three minutes a day, for each successive increase of 34°, which is about fifteen times as much as a common wire pendulum would lose under the same increase of heat; and if a metal balance had been used instead of a glass one the difference would have been still greater.
The necessity for this large amount of compensation having arisen from the variation of the elasticity of the spring, the first attempts at correcting it were by acting on the spring itself in the manner of a common regulator. Harrison's compensation consisted of a compound bar of brass and steel soldered together, having one end fixed to the watch-frame and the other carrying two curb pins which embraced the spring. As the brass expands more than the steel, any increase of heat made the bar bend; and so, if it was set the right way, it carried the pins along the spring, so as to shorten it. This contrivance is called a compensation curb; and it has often been reinvented, or applied in a modified form. But there are two objections to it: the motion of the curb pins does not correspond accurately enough to the variations in the force of the spring, and it disturbs the isochronism, which only subsists at certain definite lengths of the spring.
The compensation which was next invented left the spring untouched,
and provided for the variations of temperature by the
construction of the balance itself. Fig. 8 shows the plan of the
Fig. 8.
ordinary compensation balance. Each portion
of the rim of the balance is composed of an
inner bar of steel with an outer one of brass
soldered, or rather melted, upon it, and
carrying the weights b, b, which are screwed to
it. As the temperature increases, the brass
expanding must bend the steel inwards, and
so carries the weights farther in, and
diminishes the moment of inertia of the
balance, the decrease of rate being inversely
as the diameter of the balance-wheel. The
metals are generally soldered together by
pouring melted brass round a solid steel disk, and the whole is
afterwards turned and filed away till it leaves only the crossbar in
the middle lying flat and the two portions of the rim standing
edgeways. The first person to practise this method of uniting
them appears to have been either Thomas Earnshaw or Pierre le
Roy.
The adjustment of a balance for compensation can only be done by trial, and requires a good deal of time. It must be done independently of that for time—the former by shifting the weights, because the nearer they are to the crossbar the less distance they will move over as the rim bends with them. The timing is done by screws with heavy heads (t, t, fig. 8), which are just opposite to the ends of the crossbar, and consequently not affected by the bending of the rim; other screws are also provided round the rim for adjusting the moment of inertia and centre of gravity of the balance-wheel. The compensation may be done approximately by
