sk B shows
the height of the tide. The tide represented is a nearly high tide eight
feet above mean level.]

Utilizing these principles, a very elementary form of tidal-clock, or
tide-predicter, can be made, and for an open coast station it really
would not give the tides so very badly. It consists of a sort of clock
face with two hands, one nearly three times as long as the other. The
short hand, CA, should revolve round C once in twelve hours, and the
vertical height of its end A represents the height of the solar tide on
the scale of horizontal lines ruled across the face of the clock. The
long hand, AB, should revolve round A once in twelve hours and
twenty-five minutes, and the height of its end B (if A were fixed on the
zero line) would represent the lunar tide. The two revolutions are made
to occur together, either by means of a link-work parallelogram, or,
what is better in practice, by a string and pulleys, as shown; and the
height of the end point, B, of the third side or resultant, CB, read off
on a scale of horizontal parallel lines behind, represents the
combination or actual tide at the place. Every fortnight the two will
agree, and you will get spring tides of maximum height CA + AB; every
other fortnight the two will oppose, and you will get neap tides of
maximum height CA-AB.

Such a clock, if set properly and driven in the ordinary way, would then
roughly indicate the state of the tide whenever you chose to look at it
and read the height of its indicating point. It would not indeed be very
accurate, especially for such an inclosed station as Liverpool is, and
that is probably why they are not made. A great number of disturbances,
some astronomical, some terrestrial, have to be taken into account in
the complete theory. It is not an easy matter to do this, but it can be,
and has been, done; and a tide-predicter has not only been constructed,
but two of them are in regular work, predicting the tides for years
hence--one, the property of the Indian Government, for coast stations of
India; the other for various British and foreign stations, wherever the
necessary preliminary observations have been made. These machines are
the invention of Sir William Thomson. The tide-tables for Indian ports
are now always made by means of them.

[Illustration: FIG. 116.--Sir William Thomson (Lord Kelvin).]

[Illustration: FIG. 117.--Tide-gauge for recording local tides, a
pencil moved up and down by a float writes on