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