687. We have examined the essential features of the going parts of the
clock; to complete our sketch of this instrument we shall describe the
beautiful mechanism by which the striking is arranged. The model which
we represent in Fig. 102 is, as usual, rather intended to illustrate
the principles of the contrivance than to be an exact counter-part of
the arrangement found in clocks. Some of the details are not reproduced
in the model; but enough is shown to explain the principle, and to
enable the model to work.
688. When the hour-hand reaches certain points on the dial, the
striking is to commence; and a certain number of strokes must be
delivered. The striking apparatus has both to initiate the striking
and to control the number of strokes; the latter is by far the more
difficult duty. Two contrivances are in common use; we shall describe
that which is used in the best clocks.
689. An essential feature of the striking mechanism in the repeating
clock is the snail, which is shown at B. This piece must revolve once
in twelve hours, and is, therefore, attached to an axle which performs
its revolution in exactly the same time as the hour-hand of the clock.
In the model, the striking gear is shown detached from the going parts,
but it is easy to imagine how the snail can receive this motion. The
margin of the snail is marked with twelve steps, numbered from one to
twelve. The portions of the margin between each pair of steps is a part
of the circumference of a circle, of which the axis of the snail is the
centre. The correct figuring of the snail is of the utmost importance
to the correct performance of the clock. Above the snail is a portion
of a toothed wheel, F, called the rack; this contains about fourteen or
fifteen teeth. When this wheel is free, it falls down until a pin comes
in contact with the snail at B.
[Illustration: FIG. 102.]
690. The distance through which the rack falls depends upon the
position of the snail; if the pin come in contact with the part marked
I., as it does in the figure, the rack will descend but a small
distance, while, if the pin fall on the part marked VII., the rack will
have a longer fall: hence as the snail changes its position with the
successive hours, so the distance through which the rack falls changes
also. The snail is so contrived that at each hour the rack falls on
a lower step than it does in the preceding hour; for example, during
the hour of three o’clock, the rack would, if allowed to fall, always
drop upon the part of the snail marked III., but, when four o’clock has
arrived, the rack would fall on the part marked IV.; it is to insure
that this shall happen correctly that such attention must be paid to
the form of the snail.
691. A is a small piece called the “gathering pallet”; it is so placed
with reference to the rack that, at each revolution of A, the pallet
raises the rack one tooth. Thus, after the rack has fallen, the
gathering pallet gradually raises it.
692. On the same axle as the gathering pallet, and turning with it, is
another piece C, the object of which is to arrest the motion when the
rack has been raised, sufficiently. On the rack is a projecting pin;
the piece C passes free of this pin until the rack has been lifted to
its original height, when C is caught by the pin, and the mechanism is
stopped. The magnitude of the teeth in the rack is so arranged with
reference to the snail, that the number of lifts which the pallet must
make in raising the rack is equal to the number marked upon the step
of the snail upon which the rack had fallen; hence the snail has the
effect of controlling the number of revolutions which the gathering
pallet can make. The rack is retained by a detent F, after being raised
each tooth.
693. The gathering pallet is turned by a small pinion of 27 teeth, and
the pinion is worked by the wheel C, of 180 teeth. This wheel carries
a barrel, to which a movement of rotation is given by a weight, the
arrangement of which is evident: a second pinion of 27 teeth on the
same axle with D is also turned by the large wheel C. Since these
pinions are equal, they revolve with equal velocities. Over D the
bell I is placed; its hammer E is so arranged that a pin attached to
D strikes the bell once in every revolution of D. The action will now
be easily understood. When the hour-hand reaches the hour, a simple
arrangement raises the detent F; the rack then drops; the moment the
rack drops, the gathering pallet commences to revolve and raises up the
rack; as each tooth is raised a stroke is given to the bell, and thus
the bell strikes until the piece C is brought to rest against the pin.
694. The object of the fan H is to control the rapidity of the motion:
when its blades are placed more or less obliquely, the velocity is
lessened or increased.