Page:Cyclopaedia, Chambers - Volume 2.djvu/183

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low by too profufe Evacuations; in regard Mercurials tend to keep up or increafe the Excefs of Impetus in the Fluids, and that Over-Capacity in the fecretory Orifices,wherein the Defect of fuch a Conftitution feems to confift. See Hectic.

MERIDIAN, in Agronomy, a great Circle of the Sphere, paffing thro' the Zenith, Nadir, and Poles of the World; and dividing the Sphere into two Hemifpheres, the one Eaftem, and the other Weftern. See Circle, and Sphere.

Or, the Meridian is a Vertical Circle, as A Z B N, ('Tab. Ajironomy. Fig. 6.) patting thro' the Poles of the World, P and Q. See Vertical Circle.

It is call'd Meridian, from the Latin, Meridies, Noon, or Mid-day, by reafon when the Sun is in this Circle, it is Noon in that Place.

Meridian, in Geography, is a great Circle, as P A Q_ D, (Tab. Geography, Fig. 7.) pacing thro' the Poles of the Earth P and Q_; and any given Place, as Z. So that the Plane of the Terreftrial Meridian, is in the Plane of the Celelliai one.

Hence, 1. As the Meridian invefts the whole Earth, there are feveral Places fituated under the fame Meridian. And, 2. As it is Noon- Tide whenever the Centre of the Sun is in the "Merid.dn of the Heavens; and as the Meridian of the Earth in the Plane of the former : it follows, that it is Noon, at the fame time, in all Places fituate under the fame Meridian. 3. There are fo many Meridians on the Earth, as there are Points conceived in the ./Equator. In effect, the Meridians always change, as you change the Longitude of the Place j and may be faid to be infinite : Each feveral Place from Eall to Weil: having its feveral Meridian. See Longitude.

Fh-fi Meridian, is that from which the reft are ac- counted, reckoning from Weft to Eaft. The firft Meridian is the beginning of Longitude. See Longitude.

The fixing of a firft Meridian is a matter merely arbi- trary; and hence different Perfons, Nations, and Ages, have fix'd it differently j whence fome confufion has arofe in Geography. The Rule among the Antients was to make it pafs thro' the Place furtheft to the Weft that wasknown. But the Moderns knowing, that there is no fuch Place in the Earthascanbeefteemed the moft Wefterly j the way of computing the Longitudes of Places from one fix'd Point is much laid afide.

Ptolemy affumed the Meridian that pafTes thro' the fur- theft of the Canary Iflands as his firft Meridian 5 that being the moft Weftern Place of the World then known. After him, as more Countries were difcovered in that Quarter, the firft Meridian was removed further off. Some fix'd it to the Ifiand of St. Nicholas; Hond'tus to the Ifle of St. 'James 5 others to the Ifiand Del Corvo one of the Azores. The lateft Geographers, particularly the Dutch, have pitch 'd on the Tike of Teneriff 5 others to thelflc of Taint, another of the Canaries; and, Uftly, thcFrench, by Com- mand of their King, to the Ifiand of-Fero, another of the Canaries.

But, without much regard to any of thefe Rules, our Geographers, and Map-Makers, frequently affume the Meridian of the Place where they live, or the Capital of their Country for a firft Meridian, and thence reckon the Longitudes of their Places. '

The Aftronomers in their Calculations ufually chufe the Meridians of the Place where their Obfervations were made, for their firft Meridian, as Ptolemy at Alexandria 3 Tycho Brahe at Uranibourg 5 Riccioli at Boulogne; Mr. Flam- jieedaxthc Royal Oljfervatory at Greenwich 5 the French at theObfervatory at Park.

In the Philofophical Tranfaffions, there is a Suggeftion, that Meridians vary, in Time. This feems very probable from the old Meridian- Line in the Church of St. Petronio in bononia, which is found to vary no lefs than 8 Degrees from the true Meridian of the Place at this time j and from that of Ty xho at Uranibourg, which M. Picart obferves, va- ries 18' from the modern Meridian. If there be any thing of Truth in this Hint, Dr. Wallis fays, the Change mult arife from a change of theTerreftrial Poles (here on Earth, of the Earth's diurnal Modon) not of their pointing to this or that of the fixed Stars : For if the Poles of the diurnal Motion remain fixed 10 the fame Placeon the Earth, the Meridians which pafs through thefe Poles muft remain the fame.

But the Notion of the Changes of the Meridian is over- thrown by an Obfervation of M. Chazelies of the French Academy of Sciences, who, when in Egypt, found that the four fides of a Pyramid built 3000 Years ago, ftill look'd very exactly to the four Cardinal Points. A Pofi- tion which can never be look 'd on as fortuitous.

Meridian ofaGlobe, or Sfhere y is the Brazen Circle, in which the Globe hangs and turns. See Globe.

It is divided into four yo's, or 360 Degrees, beginning at the Equinoctial : On it, each way, from the Equinoc-

tial, on the Celeftial Globes, is counted the South and North Declination of the Sun or Stars : And on the Ter- reftrial Globe, the Latitude of Places North or South. There are two Points on this Circle, call'd the Poles t and a Diameter continued from thence through the Centre of either Globe, is call'd the Axis, of the Earth or Heavens on which they are fuppofed to turn round. See Pole and Axis.

On the Terreftrial Globes there are ufually 36" Meridians drawn; one through every tenth Degree of the Equator, or through every tenth Degree of Longitude.

The L'fes of this Circle are, to (<it the Globes to any particular Latitude; to fhew the Sun's or a Star's Decli- nation, Right Afcenfiun, greateft Altitude, &c. See Globe.

Meridian Line, an Arch, or part of the Meridian of a Place 5 terminated, each way, by the Horizon : Or a Meridian is the Interferon of the Plane of the Meridian of the Place, with the Plane of the Horizon j vulgarly call'd a. North and South Line; becaufe its Direction is from one Pole towards the other. See Meridian.

The Ufe of Meridian Lines in Aftronomy, Geography^ Dialling, £5c. is very great j and on its Exaclnefs all de- pends : Whence infinite Pains have been taken by divers Aftronomers to h-.^ve it to the laft Precifion. M. Offffm has diib'ngui/h'd himfelf by a Meridian Line drawn on the Pavement in the Church of Sr. Petronia at Boulogne, the largeft and moft accurate in the World. In the Roof of the Church, icco Inches above the Pavement, js a little Hole through which the Sun's Image, when in the Meridian falling upon the Line, marks his Progrefs all the Year. When finifhed, M.djfini, by a public Wr ting, informed the Mathematicians of Europe, of a new Oracle of .-pullo or the Sun eftabli/hed in a Temple, which might be ^ a~ fufted with entire Confidence as to ail Difficulties in Aftro- nomy. See Solstice.

To draw a Meridian Line. Knowing the South Quarter pretty nearly, obferve the Altitude F E (Tab. Ajironomy, Fig. 8.) of'fome Star on the Eall em fide thereof not far from the Meridian, HZ RN. Then, keeping the Quadrant firm on its Axis, fo as the Plummet may ftill cut the fame Degree, only directing ic to the Weftern-fidc of the Meridian, wait til) you find the Star has the fame Altitude as before, / e. i_,aftly, BitTecl: the Angle E Ceform'd by the Interfection of the t^o Planes wherein the Quadrant is placed at the time of the two Obfervations, by the right Line H R. This H R is a Meridian Line.

Or thus; On a Horizontal Plane, from the fame Centre C, (Fig. 51.) defcribe feveral Arches of Circles BA, h a t &c. And on the fame Centre C, ere ct a Style or Gnomon perpen- dicular to the Plane ACB, afoot, or half a foot long. About the 21ft of June, between the Hours of 9 andir in the Morning, and between 1 and 3 after Noon, obferve the Points B h, &c. A a, &c. wherein the Shadow of the Style terminates. BiiTect the Arches AB, ah, &c. in D d, ££?c. If then the fame right Line D E bi£fect all the Arches AB, a />, £5?r. it will be the Meridian Line fought. In regard the Extremity of the Shadow is fomewhat hard to determine, 'tis beft to have the Style flat a-top, and to drill a little Hole, noting the lucid Spot projected by it on the Arches A B and ab, inftead of the Extremity of rhe Shadow. Otherwife the Circles may be made with yellow inftead of black, &c.

Several Authors have invented particular Inftruments and Methods for the defcribing of Meridian Lines, or ra- ther for the determining equal Altitudes of the Sun in the Eaftern and Weftern Parts of the Heavens; as Mr. Grey, Mr. Derham, &c. in the Philof. Tranf. But as the former of the Methods above delivered, fuffices for Aftro- nomical Obfervations, and the latter for more ordinary Oc- cafions, we /hall forbear to giveany Defcriptions thereof.

From what has been ftiewn, it is evident that whenever the Shadow of the Style covers the Meridian Line, the Centre of the Sun is in the Meridian j and therefore it is then Noon. And hence the TJfe of a Meridian Line in ad- jufting the Motion of Clocks, &c. to the Sun. Hence, alfo, if the Meridian Line be biffected by a right Line OV, drawn perpendicularly through the Point C j O V will be the Interferon of the Meridian, and firft Vertical j and, confequently, O will /hew the Eaft Point, and V the Weft, Lallly, if a Style be erected perpen- dicularly in any other horizontal Plane, and a Signal be given when the Shadow of the Style covers the Meridian Line drawn in another Plane, noting the Apex or Extre- mity of the Shadow projected by the Style, a Line drawn from that Point thro' that wherein the Style is rais'd will be a Meridian Line. See Meridian Altitude.

Meridian Line on a Dial, is a right Line arifing from an Interferon of the Meridian of the Place with the Plane of the Dial. See Dial.

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