Page:Cyclopaedia, Chambers - Volume 2.djvu/869

 T I D

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T I D

Its lower Extremity, which is much fmaller than its upper, has a remarkable Procefs, which forms the inner Ankle 5 and a pretty large Sinus, divided in the Middle by a fmall Protu- berance; theSinus receives the convex Head of the Aftragalus, and the Protuberance is received into the Sinus, in the convex Head of the fame Bone.

It has another fhallow Sinus in the Side of its lower End, which receives the Fibula.

Mr. Chefelien gives an Inftance of a Boy of Seven Tears of Age, where both the Epiphyfes at the upper End of the Tibice, were fb far feparated, that not mote than Half each Tibia was join'd to Half the Epiphyiis ; 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 young ; which is among them (as he obferves) too common a Practice.

TIBIALIS or Tibiveus, in Anatomy, a Name given two Mufcles of the Leg ; diftinguifli'd by Amicus and 'Poflicus.

The Tibialis Anticus, fprings from the exterior Procefs of the Tibia, and becoming gradually broad and flefliy about the middle of the Tibia, down the Fore-part of which it runs, is contracted again, into a flendcr, fmooth Tendon, which paffes under the Ligamentum Annulare, and is partly inferted to the Qs Cuneifbrme 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, fli-ong 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.

TICHONIC or/to or Hypothefis. See Tychonic.

TIDES, Two periodical Motions of the Waters of the Sea, call'd the Flux and Reflux ; or the Ebb and Flow. See Sea, Flux, Ebb, ?£c.

When the Motion of the Water is again!! the Wind, 'tis call'd a Windward Tide ; when Wind and Tide go the fame Way, Leeward 'tide : When it runs very fttong,' '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 lafts ; then to flay at Anchor all the Time of contrary Tide 5 and thus to let in again with the Return of the firft Tide.

It is laid to flow Tide and Half Tide, when the Tide runs three 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-fliorc at 12, it will not be fb in the Offing till Three. If it Ebb and Flow longer, they fay it runs Half tide and Half-quarter.

When the Moon is in the Firft and Third Quarter, i. e. when Hie is New and Full, the Tides are High and Swift, and called Spring Tides ; when lfie is in the fecond and laft Quarter, the Tides are lower and flower, and call'd Neap Tides. See Neap, &c.

ThanomeM of the Tides.

The Sea is obferv'd to flow, for certain Hours, from South to- wards North ; in which Motion, or Flux, which lafts about Six Hours, the Sea gradually fwells 5 fb that entering the Mouths of Rivers, it drives back the River-waters towards their Heads or Springs. See River, l$c.

After a continual Flux of Six Hours, the Sea feems to reft for about a Quarter of an Hour ; after which it begins to ebb or retire back again {romNorth to South, for Six Hours more: In which Time, the Water finking, the Rivets refume their natural Courfe. After afeeming Paufeof 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 50 Minutes, fo that thttides return later, and later each Day, by jo Minutes or f of an Hour, j Minutes.

Now, 12 Hours 50 Minutes is a Lunar Day ; i. e. the Moon paffes the Earth's Meridian later and later each Day by 50 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 ffie paffes the Horizon, both the Eaftem and Weftern Point thereof. See Moon.

This further Agreement we obferve between the Moon and the Sea ; that the Tides, though conftant, are not equal ; but are greateft, t when the Moon is in Conjunction, or Oppo- fition to the Sun, and leaft when in Quadrate thereto.

Laftly, thofe Tides are the greateft, 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 Coafts of Europe ; only that the Tides are fo much the lefs, and happen the later, as the Coafts are the more Northerly.

Thefe Pharomena of the Tides are admirably accounted for, from the Principle of Gravitation. All we require to

their Solution, is, that the Earth and Moon, and every Particle thereof, mutually gtavitate towards each other ; the Reafonablenefs of which Affumption, fee under the Article Gravitation.

Indeed the fagacious Kepler long ago, conjectured this to be the Caufe of the Tides : " If, fays he, the Earth ceas'd " to attraft its Waters towards itfelf ; all the Water of the " Ocean would rife and flow into the Moon : The Sphere of " the Moon's Attraction extends to our Earth, and draws up " the Water." Thus thought Kepler in his Introd. ad theor. Mart. This Surmize, for it was then no more, is now- abundantly verified, in the following Theory, deduced by Dr. Halley, from the Newtonian Principles.

theory of the Tides.

1° As the Surface of the Earth and Sea is naturally globu- lar ; if we fuppofe the Moon A (Tab. Geography Fig. 61) perpendicularly over fome Part of the Surface of the Sea, as E; 'tis evident the Water E, which is now neareft the Moon, will gravitate towards it more than any other Part of the Earth and Sea in the Hemifphere FPH.

The Water in E, therefore, muft by this means, be rais'd towards the Moon, i. e. it will be lighter than ufual, and of confequence will fwell in E.

For the fame Reafon the Water in G being the moft 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, muft approach lefs rowards the Moon, than any other Part of the Globe, i. e. it muft be rais'd the contrary Way ; as being lighter than ufual, and will therefore fwell in G.

By this means, the Surface of the Ocean muft neceffarily form itfelf into a Spheroidal, or Oval Figure, whofe longer Diameter is EG; its fhorter FH. And thus, the Moon fluffing her Ppfition in her Diurnal Motion round the Earth ; this Oval of Water muft fhift with her j by which means are effected thofe two Floods and Ebbs, obfervable every 25 Hours.

2° Since, in the Conjunctions and Oppositions 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 Tides are greateft in the Syzygies, and leaft in the Quadratures.

In effect, there are two Tides every natural Day, from the Action 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 fb imiiienfe a Diftance, that the Earth's Semi-diameter bears no Proportion thereto.

Hence, the different 7?i« depending on the particular Acti- ons of the Sun and Moon are not diftinguifli'd, but confound- ed. The Lunar Tide is fomewhat changed by the Action of the Sun ; and this Change varies every Day, by reafon of the Inequality between the Natural and Lunat Day. See Day.

3 Since the greateft Tides about the Equinoxes (viz. thofe happening in the Syzygies) arife from the Sun and Moon being in the Equinoctial ; and thofe about the Solftices, from the Sun and Moon being in the Tropicks ; for this Reafon, thofe greateft Tides about the Equinoxes are greater than thofe about the Solftices, fince the greater the Circle is, wherein the Waters turn, the greater is their Agitation. And if the Moon flood ftill in the Pole, the Spheroid would become immoveable about the Pole, and the high Water be fix'd therein.

4°. Since the Tides are fomewhat chang'd by the Librati- on of the Waters, which ufe to retain a Motion imprefs'd on them for fome time ; for this Reafon the higheft Tides arc not ptecifely in the very Conjunct ion and Oppofition of the Moon, but two or three Tides afterwards.

5, Since the Sun is fomewhat neater the Earth in Winter than in Summer 5 hence it is, that the greateft Equinoctial 'tides are obferv'd to be a little before the Vernal Equinox, and a little after the Autumnal one.

6°. Since the greateft of the two Tides 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 atifing ftom the Moon above the Horizon 5 when the Sun is in the Southern Signs, the greateft is that atifing from the Moon below the Ho- rizon.

1°. Such would the 'tides regularly be, if the Earth were cover'd with Sea very deep 5 but by Reafon of the Shoal- nefs of fome Places, and the Narrownefs of the Straits in others, by which the Tides ate propagated, there arifes a great Diverfity in the EffecTr, not to be accounted for, with- out an exact Knowledge of all the Circumftances of the Places j as the Pofition of the Land, and the Breadth and

Depth