_THEORY OF TIDES_, ILLUSTRATED.
This Scene also consists of the Sun, the Earth, and the Moon. But the
intention is to shew how the Earth and Moon agitate each other round
their common centre of gravity, causing two tides every 25 hours. It is
a circumstance truly astonishing, to see in calm weather, and under a
serene sky, the violent agitation of the great fluid mass of the ocean,
whose waves roll against the shore with so grand impetuosity.—This
spectacle invites to reflection, and rouses a desire to penetrate
into its cause: hence the Earth’s THREE-FOLD motion appears in this
scene:—that, on its axis, to produce day and night: that, round the
Sun, to produce the year and our seasons; and that, round the centre
of gravity with the Moon, to produce spring and neap tides, by their
combined and opposite influences. The Moon is so near the Earth (240,000
miles at a medium) in comparison of the Sun (near 100 millions of miles)
that the Moon’s attraction on the waters of the ocean and on the air of
our atmosphere (for there are tides in both) is to that of the Sun as
ten is to three. So at the change of the Moon, the attraction of the Sun
and Moon being in the same direction, a power of thirteen influences the
sea, and we have SPRING tides; but at the quarters of the Moon, the two
luminaries counteract the attractions of each other, so the Sun’s power
of three being taken from the Moon’s of ten, leaves only seven operating
upon the sea, and NEAP tides take place.
A tumbler filled with water, may be whirled by a string vertically round
the head, without any danger of the water falling out of it. Those parts
of the Earth that come successively OPPOSITE to the Moon, perform a much
larger circle round the centre of gravity, than the parts immediately
UNDER the Moon: hence the waters opposite the Moon are THROWN OFF, as it
were, by their centrifugal motion, and rise above the common level, as
well as the waters exposed to the Moon’s immediate attraction; thus two
tides are produced in 25 hours, opposite to each other; and by the Earth
turning through those protuberances, its waters rise and fall.
The Sun would produce two small, but similar tides, if the Earth had no
Moon; therefore at the full of the Moon the Sun’s centrifugal tide being
reinforced by the Moon’s attraction, and the Moon’s centrifugal tide
being also assisted by the Sun’s attraction, spring tides take place at
the full, as well as at the change, of the Moon.
☞ This Scene also receives collateral assistance, exhibiting
and explaining the setting of the tides in all the large oceans
and the principal seas of the Earth.