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Its lower Extremity, which is much fmallr than its upper,
has a remarkable Procefs, which forms the inner Ankle; and
a pretty large Sinus, divided in the Middle by a fmall Protu-
berance the Sinus receives the con'vex Head of the Affragalus,
and the Protuberance is received into the Sinus, in the convex
Head of the fame Bone.
It has another Ihallow Sinus in the Side' of its lower End,
which receives the Fibula.
Mr. Chefelden gives an Inflance of a Boy of Seven Years of
Age, where both the Epiphyfes at the upper End of the
Ttbia, were fo far feparated, that not more than Half each
ibia was join'd to Half the Epiphyfis; which made his
Legs wholly ufelefs. This had been occofion'd, by the Nurfe's
holding him out to Stool by the Heels and Back, when very
ong; which is among them (as he obferves) too common a
Fraruice.
TIBIALIS or TIBIAUS, in Anatomy, a Name given two
Mufcles of the Leg; diftinguifhd by Adnticus and 'Poflicus.
The Tibialis Anticus, fprings from the exterior Procefs of
the Tibia, and becoming gradually broad and flefhy about the
middle of the Tika, down the Fore-part of which it runs, is
contra&ed again, into a flender, fmooth Tendon, which paffies
under the Ligamentum Annulare, and is partly inferted to the
Os Cuneiforme Majus, and partly to the Bone of the Metatar-
fus, that fupports the great Toe. Its Office is to draw the
Foot up.
Tibialis Poflicus, is derived from both Bones of the Tibia,
and from the Ligament that bends them together; and runs
with a fmooth, ftrong Tendon through the Sinus on the inner
Malleolus, under the Annular Ligament, to the Infide of the
Os Naviculare.
Its Office is to draw the Foot inwards. From the Ufe Sailors
make of it in Sailing, it is alfo called the Nauticus.
TICHONICSyylem or Hypothefis. See TYCHONIC.
TIDES, Two periodical Motions of the Waters of the Seas
call'd the Flux and Reflux; or the Ebb and Flow. See Sn .,
FLUX, EBB, Tc.
When the Motion of the Water is againft the Wind, 'tis
call'd a Windaward Tide; when Wind and Tide go the fame
Way, Leeward Tide: When it runs very firong, 'tis a Tide-
gate.
To Tide it over or up into any Place, is to go in with the
Tide, either Ebb or Flood, as long as that lafis; then to flay
at Anchor all the Time of contrary Tide; and thus to fet in
again with the Return of the firft Tide.
It is Paid to flow ride and Half Tide, when the Tide runs
thtee Hours in the Offing, longer than it does by the Shore:
but, by longer, they don't mean more Hours; but, that-if it be
high Water a-fhore at 12, it will nof be fo in the Offing till
Three. If it Ebb and Flow longer, they fay it runs Half
fide and Haifquarter.
When the Moon is in the Firff and Third Quarter, i. e.
when She is New and Full, the Tides are High and Swift,
and called Spring Tides; when ifhe is in the fecond and lafi
Quarter, the Tides are lower and flower, and call'd Neap
Tides. See NEAP, &-c-
Phenomema of the TiDEs.
The Sea is obferv'd to flow, for certain Hours, from South to-
wards North; in which Motion, or Flux, which lafis about Six
Hours, the Sea gradually fwells; fo that entering the Mouths
of Rivers, it drives back the River-waters towards their Heads
or Springs. See RIVER, &C.
After a continual Flux of Six Hours, the Sea feems to rell
for about a Quarter of an Hour; after which it begins to ebb
or retire back again from North to South, for Six Hours more:
In which Time, the Water finking, the Rivers refume their
natural Courfe. After a feeming Paufe of a Quarter of an Hour,
the Sea again begins to flow as before, and thus alternately.
Thus does the Sea ebb twice a-day, and flow as often; but
not in the fame Hours thereof. The Period of a Flux and Re-
flux IS 12 Hours so Minutes, fo that the Tides return later, and
later each Day, by so Minutes or - of an Hour, S Minutes.
Now, i 2 Hours 50 Minutes is a Lunar Day; i. e. the Moon
paffes the Earth's Meridian later and later each Day by 5o
Minutes. So that the Sea flows as often as the Moon paffes
the Meridian, both the Arch above, and that below the
Horizon; and Ebbs as often as fhe palfes the Horizon, both
the Faftlemand Weflern Point thereof. See MOON.
This further Agreement we obferve between the Moon and
the Sea; that the Tides, though conflant, are not e  ual
but are greatef , when the Moon is in Conjunaion, or Oppo-
fition to the Sun, and leafp when in Quadrate thereto.
Lafily, thofe Tides are the greateff, which happen in the
New and Full Moon, at the Times of the Equinoxes.
Add, that the fame Things are obferved throughout moft
of the Coafis of Europe; only that the Tides are fo much
the lefs, and happen the later, as the Coafis are the more
NThey Phanomena of the Tides are admirably accounted
for, from the Principle of Gravitation. All we require to


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their Solution, is, that the Earth and Moon, and every
Particle thereof, mutually gravitate towards each other; the
Reafonablenefs of which Afflumptio'n, fee under the Article
GRAVITATION.
Indeed the Sagacious Kepler long ago, conjeflured this to
be the Caufe of the Tides: " If, fays he, the Earth ceas'd
l to attra& its Waters towards itfelf; all the Water of the
Ocean would rife and flow into the Moon: The Sphere of
" the Moon's Attraaion extends to our Earth, and draws up
" the Water."  Thus thought Kepler in his Introd. ad 7Iheor.
Mart. This Surmize, for it was then no more, is now
abundantly verified, in the following Theory, deduced by
Dr. Halley, from the Neiwtonian Principles.
T'heory of the TxnDS.
iP As the Surface of the Earth and Sea is naturally globu-
lar; if we fuppofe the Moon A (Tab. Geography Fig. 6.)
perpendicularly over fome Part of the Surface of the Sea, as
E; 'tis evident the Water E, which is now nearedi the Moon,
will gravitate towards it more than any other Part of the
Earth and Sea in the Hemifphere FP H.
The Water in E, therefore, muff by this means, be rais'd
towards the Moon, i. e. it will be lighter than ufual, and of
consequence will fwell in E.
For the fame Reafon the Water in G being the mofi remote
from the Moon, will gravitate towards the fame, rather than
any other Part of the Earth or Sea in the Hemifphere F G H.
The Water here, therefore, muff approach lefs towards
the Moon, than any other Part of the Globe, i. e. it muff be
rais'd the contrary Way i as being lighter than ufual, and will
therefore fwell in G.
By this means, the Surface of the Ocean muff neceffarily
form itfelf into a Spheroidal, or Oval Figure, whofe longer
Diameter is E G; its fhorter F H. And thus, the Moon
fhifting her Pofition in her Diurnal Motion round the Earth;
this Oval of Water muff 1hift with her ; by which means
are effeded thofe two Floods and Ebbs, obfervable every
25 Hours.
20 Since, in the Conjunaions and Oppofitions of the Sun
and Moon, the Gravitation of the Water to the Sun con-
fpires with its Gravitation towards the Moon; but in the
Quadratures, the Water rais'd by the Sun is deprefs'd by the
Moon: Hence it is that the lides are greatefl in the Syzygies,
and leaft in the Quadratures.
In effe&, there are two Tides every natural Day, from the
Adion of the Sun, as there are in the Lunar Day from that
of the Moon; all govern'd by the fame Laws: Only thofe
caus'd by the Sun, are much lefs than thofe of the Moon;
becaufe though the Sun be Ten thoufand times bigger than
both the Earth and Moon, yet he is at fo immenfe a
Diffance, that the Earth's Semi-diameter bears no Proportion
thereto.
Hence, the different Tides depending on the particular A6di-
ons of the Sun and Moon are not diffinguiflh'd, but confound-
ed. The Lunar Tide is fomewhat changed by the Aaion of
the Sun; and this Change varies every Day, by reafon of the
Inequality between the Natural and Lunar Day. See DAY.
30 Since the greateff Tides about the Equinoxes (viz. thofe
happening in the Syzygies) arife from the Sun and Moon
being in the Equinodlial; and thofe about the Solffices, from
the Sun and Moon being in the Tropicks; for this Reafon,
thofe greateft Tides about the Equinoxes are greater than
thofe about the Solflices, fince the greater the Circle is,
wherein the Waters turn, the greater is their Agitation.
And if the Moon flood fill in the Pole, the Spheroid would
become immoveable about the Pole, and the high Water
be fix'd therein.
40. Since the Sides are fomewhat chang'd by the Librati-
on of the Waters, which ufe to retain a Motion imprefs'd on
them for lome time; for this Reafon the higheff Tides are
not precifely in the very Conjunaion and Oppofition of the
Moon, but two or three Tides afterwards.
50. Since the Sun is fomewhat neater the Earth in Winter
than in Summer; hence it is, that the greateff Equinoaial
Tides are obferv'd to be a little before the Vernal Equinox,
and a little after the Autumnal one.
60. Since the greateff of the two T1ides happening in every
Diurnal Revolution of the Moon, is that wherein the Moon
is neareft the Zenith, or Nadir: For this Reafon, while
the Sun is in the Northern Signs, the greater of the two
Diurnal Tides in our Climates, is that arifing from the Moon
above the Horizon; when the Sun is in the Southern Signs,
the greateft is that arifing from the Moon below the Ho-
rizon.
70. Such would the Tides regularly be, if the Earth were
coveed with Sea very deep; but by Reafon of the Shoal-
nefs of fome Places, and the Narrownefs of the Straits in
others, by which the Tides are propagated, there arifes a
great Diverfity in the EfFea', not to be accounted for, with-
out an exadl Knowledge of all the Circumflances of the
Places i as the Pofition of the Land, and the Breadth and
Depth


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