441. The TIDE-DIAL. The outside parts of this Machine consist of, 1. An
eight-sided Box, on the top of which at the corner is shewn the Phases
of the Moon at the Octants, Quarters, and Full. Within these is a Circle
of 29-1/2 equal parts, which are the days of the Moon’s age accounted
from the Sun at New Moon round to the same again. Within this Circle is
one of 24 hours divided into their respective Halves and Quarters. 2. A
moving elliptical Plate painted blue to represent the rising of the
Tides under and opposite to the Moon; and has the words, _High Water,
Tide falling, Low Water, Tide rising_, marked upon it. To one end of
this Plate is fixed the Moon _M_ by the Wire _W_, and goes along with
it. 3. Above this elliptical Plate is a round one, with the Points of
the Compass upon it, and also the names of above 200 places in the large
Machine (but only 32 in the Figure to avoid confusion) set over those
Points on which the Moon bears when she raises the Tides to the greatest
heights at these Places twice in every lunar day: and to the North and
South Points of this Plate are fixed two Indexes _I_ and _K_, which shew
the times of High Water in the Hour Circle at all these places. 4. Below
the elliptical Plate are four small Plates, two of which project out
from below its ends at New and Full Moon; and so, by lengthening the
Ellipse shew the Spring Tides, which are then raised to the greatest
heights by the united attractions of the Sun and Moon § 302. The other
two of these small Plates appear at low water when the Moon is in her
Quadratures, or at the sides of the elliptic Plate, to shew the Nepe
Tides; the Sun and Moon then acting cross-wise to each other. When any
two of these small Plates appear, the other two are hid; and when the
Moon is in her Octants they all disappear, there being neither Spring
nor Nepe Tides at those times. Within the Box are a few Wheels for
performing these Motions by the Handle or Winch _H_.
[Illustration: Plate XIII.
_J. Ferguson inv. et del._ _J. Mynde Sculp._]
Turn the Handle until the Moon _M_ comes to any given day of her age in
the Circle of 29-1/2 equal parts, and the Moon’s Wire _W_ will cut the
time of her coming to the Meridian on that day, in the Hour Circle; the
XII under the Sun being Mid-day, and the opposite XII Mid-night: then
looking for the name of any given place on the round Plate (which makes
29-1/2 rotations whilst the Moon _M_ makes only one revolution from the
Sun to the Sun again) turn the Handle till _that_ place comes to the
word _High Water_ under the Moon, and the Index which falls among the
Afternoon Hours will shew the time of high water at that place in the
Afternoon of the given day: then turn the Plate half round, till the
same place comes to the opposite High Water Mark, and the Index will
shew the time of High Water in the Forenoon at that place. And thus, as
all the different places come successively under and opposite to the
Moon, the Indexes shew the times of High Water at them in both parts of
the day: and when the same places come to the Low Water Marks the
Indexes shew the times of Low Water. For about two days before and after
the times of New and Full Moon, the two small Plates come out a little
way from below the High Water Marks on the elliptical Plate, to shew
that the Tides rise still higher about these times: and about the
Quarters, the other two Plates come out a little from under the Low
Water Marks towards the Sun and on the opposite side, shewing that the
Tides of Flood rise not then so high, nor do the Tides of Ebb fall so
low, as at other times.
By pulling the Handle a little way outward, it is disengaged from the
Wheel-work, and then the upper Plate may be turned round quickly by hand
so, as the Moon may be brought to any given day of her age in about a
quarter of a minute.
[Sidenote: The inside work described.