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takes a long time, and there must be some special
machinery to keep it going all that time, and also
to prevent the winding man from accidentally stop-
ping the striking by going on winding when the
clock ought to strike. The winding of the going
| part of this clock takes ten minutes, but the wind-

ing of the two striking parts together (if done by
| ene man) takes ten hours. The going part is main-
_ tained by a peculiar self-acting contrivance which
exists in no other clock, and can neither run itself
out of gear, nor fail in strength, however the man
may loiter over the winding ; and that is done
without the addition of a single wheel to those
which would otherwise have been required ; for so

| to increase the power of the winder.

The striking parts being driven by weights of a
| ton and a half each, with a fall of 170 feet, require
two multiplying wheels to wind them up; and
| these again have a peculiar contrivance which pulls
_up the winding man about a minute before the
clock wants to strike and makes him wait till it
| has done, which happens about twenty times in
| the course of each winding. ‘The striking weights
are hung by steel wire ropes half an inch thick.

fall of 170 feet, and the clock will go 81 days, to
provide for the possible forgetting of a day. But
the striking parts have to be wound up twice a
week, as it was impossible to get power enough
without, except by still larger machinery in-
volving much more friction ; so that the winding
would have taken not a shorter, but a longer time
altogether, and in fact, each part could not have
been wound upinaday. It was considered whether

it was worth while to apply steam or water power |
to it, but it was calculated that it would on the |

whole cost more, including the additional machinery,
and the presence of a man could not safely be dis-
pensed with.

The effect of wind and snow and the variations
of friction are cut off from reaching the pendulum,
which is the ultimate time-regulator in all clocks,
by a very simple apparatus, which goes by the name
of the gravity escapement, invented by Mr. Deni-
son, and now largely used in church clocks, and
also in some “regulators ” or astronomical clocks,
though it is not so valuable for them, as they are
not exposed to such disturbing causes. Its opera-
tion here is equivalent to the raising of an ounce
weight about an inch by the clock at every beat of
the pendulum, and letting it fall with the pendulum,
which has nothing to do with raising it. There-
fore, whether it is raised quickly or slowly, or with
more or less force by the clock, its action on the
pendulum is always just the same ; and as it acts
almost without friction, the pendulum is always
unter the influence of a practically constant force,

large a clock requires at least one multiplying wheel.

| The going weight is 1} ewt., but has the same |

THE GREAT CLOCK OF WESTMINSTER.

and therefore always swings through the same are,
of 22° on each side of zero, and always in the same
time, subject only to atmospheric variations, which
may affect it slightly.

The variations of temperature are provided
against in the usual way by ‘‘ compensation.”
For that purpose the pendulum consists first of an
iron rod about 15 feet long, with a nut at the
bottom, on which rests a thick zine tube about 10
feet long; and from the top of that is hung a
larger thin iron tube of the same length, on the
bottom of which is another large nut or ring, on
which rests a large cylinder or bob of cast iron,
with a domed top, weighing above 4 ewt. The
10 feet of zine expand upwards as much as the 25
feet of iron downwards by any increase of heat,
and so the pendulum is always kept of the effective
length requisite to vibrate in two seconds. The
whole pendulum weighs 685 lhs., and is hung by a
steel spring, 3 inches wide and a 60th of an inch
thick, to a large iron frame or “cock” built into
the wall behind the clock. This is the heaviest
pendulum in the world, though not the longest.
Some old church clock pendulums are as long as
56 feet, vibrating in four seconds. But though
length as well as weight of pendulum diminishes
the errors of a clock, very long pendulums are
subject to draughts and inequalities of temperature
and other difficulties of compensation, and they are
never used now.

This pendulum is regulated or made to go faster
or slower by adding or taking off very small
weights, which lie on a collar half way down the
pendulum. In round numbers, the ten-thousandth
part of the weight of the pendulum added there,
or an ounce in this case, accelerates it a second a
day. When the clock is two or three seconds
slow or fast of Greenwich time, it is set right by
adding or taking off for half an hour or so a
heavier weight, which lies on the pendulum at the
same place, without ever stopping or disturbing it,
as all other methods do, from which it does not
recover for a long time. ;

This clock is subject to a test of its going far
more severe than any other public clock ever was;
for it reports by electric telegraph its own time of
striking to the Royal Observatory at Greenwich
twice a day; and a return signal is sent from
Greenwich for the information of the men who go
to wind it up. The Astronomer Royal every year
records its performance in his report to the Board
of Visitors of the Observatory; and it appears
from those reports that it is very seldom as much
as three seconds wrong, though it is not the
custom to meddle with it until the error has
accumulated to that amount. So that it may be
practically always relied on within two seconds,
and its accuracy has immensely exceeded that

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