Repeating clocks and watches may be considered as instruments
for registering time, which communicate their information only
when the owner requires it, by pulling a string, or by some
similar application.
An apparatus has recently been applied to watches, by which
the hand which indicates seconds leaves a small dot of ink on the
dial-plate whenever a certain stop or detent is pushed in. Thus,
whilst the eye is attentively fixed on the phenomenon to be
observed, the finger registers on the face of the watch-dial the
commencement and the end of its appearance.
75. Several instruments have been contrived for awakening the
attention of the observer at times previously fixed upon. The
various kinds of alarums connected with clocks and watches are of
this kind. In some instances it is desirable to be able to set
them so as to give notice at many successive and distant points
of time, such as those of the arrival of given stars on the
meridian. A clock of this kind is used at the Royal Observatory
at Greenwich.
76. An earthquake is a phenomenon of such frequent occurrence,
and so interesting, both from its fearful devastations as well as
from its connection with geological theories, that it becomes
important to possess an instrument which shall, if possible,
indicate the direction of the shock, as well as its intensity.
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