Page:Cyclopaedia, Chambers - Volume 2.djvu/142

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may be reprefented in one My ; if inftead of the Plane of the firft Meridian, fome other Plane parallel to it, but very near the Eye, be taken ; for by this means the entire Pa- rallels and Meridians will be delcribed. But as this diftorts the Face of the Earth too much, it i s feldom ufed ; and we rather make the two Hemifpheres in two diltinct Tables. One great Advantage in this Projection, is, that it repre- fents the Longitudes and Latitudes of Places, their Diftance from the Pole and from the Equator, almoft the fame as they are on the Earth. Its Inconveniences are, rhat it makes the Degrees of the Equatorunequal, being the greater as they are nearer the firft Meridian DAB, or its oppofite BCD; and for this Reafon equal Tracts of the Earth are reprefented unequal ; which Defefl may be infome mea- fure remedy'd by removing the Eye far from the Earth. Laftly, the Diftances of Places, and Situation with re- gard to each other, cannot be well determined mMaps of this Projection.

ProjeBionofMapsonthe Flans cf the Horizon, or wherein any given Place /hall be the Centre, or Middle. Suppofe, for inflance, 'tis defired to have London the Centre of the Map. Its Latitude we'll fuppofe to be 51. jz Min. The Eye is placed in the Nadir. The tranfparent Table is the Plane of the Horizon, or fome other Plane, if 'tis de- fired to reprefent more than an Hemifphere. Take then the Point E (Fig. 4.) for London, and from this, as a Centre, defcribe the Circle A BCD to reprefent the Horizon, which you are then to divide into four Qua- drants, and each of thefe into 90 Degrees. Let the Dia- meter B D be the Meridian, B the Northern Quarter, D the Southern ; the Line of EquinoSial Eaft and Welt, Ihews the firft Vertical, A the Weft, C the Eaft, or a Place 90 deg. from the Zenith in the firft Vertical. All the Verticals are reprefented by right Lines drawn from the Centre E to the feveral Degrees of the Horizon. Di- vide BD into 1 80 deg. as in the former Methods; the Point in EB reprefenting 51 deg. 32 min. of the Arch B C, will be the Projection of the North Pole, which note with the Letter P. The Point in E D reprefenting 51 deg. 30 min. of the Arch DC, (reckoning from C towards D) will be the Projection of the Interaction of the Equator and Meridian of London, which note with the Letter Q, and from this, towards P, write the Numbers of the De- grees, 1, a,?,ts?c. As alfo from Qjowards D, and from B towards P, viz. 51, 52, 53, i£c.

Then taking the correfponding Points of equal Degrees, •viz. 99 and $y, 88 and SS.gJc. about thofe, as Diameters, defcribe Circles, which will reprefent Parallels, or Circles of Latitude, with the Equator, Tropics, and Polar Circles. For the Meridians, firft defcribe a Circle thro the three Points A, P, C. This will reprefent the Meridian 90 deg. from London. Let its Centre be M in BD (continued to the Point N, which reprefents the South Pole) PN being the Diameter, thro M draw a Parallel to A C, viz. F H, continued each Way to K and L. Divide the Circle PHNF into 3o~odeg. and from the Point P draw right Lines to the feveral Degrees, cutting K F H L thro the feveral Points of Interferon, and the two Poles P, N, as thro, three given Points, defcribe Circles reprefenting all the Meridians. The Centres for defcribing the Arches will be in the fame K, L, as being the fame, that are found by the former Interfection ; but are to be taken with this Caution, that for the Meridian next B D N to- wards A, the moft remote Centre towards L be taken for the 2d, the 2d from this, g£e. The Circles of Longitude and Latitude thus drawn, infert the Places from a Table as before directed.

Projection of Maps on the Plane of the Meridian. This Projection is taught by Ptolemy, and recommended by him as proper for that part of the Earth then known. In the Equator and Parallels are Arches of Circles, and in the Meridians, Arches of Ellipfes ; the Eye hanging over the Plane of that Meridian which paffes over the middle of the inhabited World. But in regard the Defcription of thefe Ellipfes is fomewhat perplexing, and becaufe this Method feems only calculated for a part of the Earth ; 'tis not now ufed.

There is a fecond Method fomething a-kin to it, which reprefents the Circles of Latitude by right Lines, and the Meridians by Arches of Ellipfes; as mail be the Cafe, if Lines be conceived to fall from the feveral Points of each Hemifphere, perpendicularly on the Plane of the firft Meridian, and the Eye be fuppofed at an infinite diftance from the Earth ; fo that all the Rays emitted from the Places of the Earth to it, may be accounted Parallels as well as Perpendiculars to the Plane of the firft Meri- dian.

Rectilinear Maps, thofe wherein both the Meridians and Parallels are reprefented by right Lines, which by the Laws of Ferfpective is impoffible ; in regard rhere can no fuch Pofition be aflign'd the Eye and the Plane, as that the Circles both of Longitude and Latitude /hall be right

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Lines. In the firft Method laid down, the Meridians ate right Lines, but the Parallels are Circles. In the fifth the Parallels are right Lines, and the Meridians Ellipfes.' In all other perfpeftive Methods, both Kinds of Circles are Curve ; one Method indeed muft be excepted, wherein the Meridians are right Lines, and the Parallels Hyperbo- la's } as when the Eye is placed in the Centre of the Earth, and the Plane, thro which it is view'd, is parallel to the firft Meridian: but this Method is rather pretty than ufeful. ReBilinear Map ate chiefly ufed in Naviga tion, to facilitate the Lftimation of the Shin's Way. See Chart. j °

CanjlrttBion of particular or fpecial Maps.

Particular Maps of large Traits, as Europe, Jfia, Jfrica, and America, are projected after the fame Manner as Ge- neral ones; only let it be obferved, that for different Parts, different Methods be chofen. Africa and America, for inftance, in regard the Equator paffes thro them, can- not be conveniently projected by the firft Method, but much better by the fecond. Europe and sifia are moft conveniently reprefented by the third; and the polar Parts, or the frigid Zones, by the firft.

To begin then, draw a right Line on your Plane or Pa- per, for the Meridian of the Place over which the Eye is conceiv'd to hang, and divide it into Degrees, as before which will be Degrees of Latitude. Then' from the Tables take the Latitude of the two Parallels, which terminate each Extreme. The Degrees of thefe Latitudes are to be noted in the Meridian ; and thro them draw Perpen- diculars, bounding the Map towards North and South. This done, Meridians and Parallels are to be drawn to the feveral Degrees, and the Places to be inferted, till the Map is complear.

_ Particular Maps %f left Extent. In Maps of fmaller Por- tions of the Earth, the Geographers take another Me- thod. Firft, a tranfverfe Line is drawn at the bottom of the Plane, to reprefent the Latitude, wherein the Souther- moft part of the County to be exhibited, terminates. In this Line, fo many equal Parts are taken, as that Country is extended in Longitude. On the middle of this fame Line erect a Perpendicular, having fo many Parts as there are Degrees of Latitude between the Northern and Southern Limits of the Country. How big thefe Parts are to be, may be determin'd by the Proportion of a De- gree of a great Circle to a Degree of the Parallel repre- fented by the tranfverfe Line at bottom. See Degree. Thro' the other Extreme of this Perpendicular, draw an- other Perpendicular, or a Parallel to the Line at bottom, in which are to be as many Degrees of Longitude, as in the lower Line, and thefe, too, equal to thofe other, un- lefs the Latitudes happen to be remote from each other, or from the Equator. But if the loweft Parallel be at a confiderable diftance from the Equinoctial, or if the Latitude of the Northern Limit go much beyond that of the Southern ; the Parts or Degrees of the upper Line muft not be equal to thofe of the lower, but lefs, according to the Proportion which a Degree of the more Northern Parallel, has to a Degree of the more Southern : Which fee, as before, under the word Degree.

After Parts have been thus determin'd, both on the upper and lower Line, for the Degrees of Longitude ; right Lines muft be drawn thro' the beginning and end of the fame Number, which Lines reprefent Meridians : then, thro the feveral Degrees of the Perpendicular erec- ted on the middle of the firft tranfverfe Line, draw Lines parallel to that tranfverfe Line. Thefe will reprefent Parallels of Latitude. Laftly, at the Points wherein the Meridians of Longitude and the Parallels of Latitude con- cur, infert the Places from a Table, as before directed.

For Maps of Provinces, or fmall Trafls, as Parilries, Mannors, gfc. we ufe another Method, more fure anri accurate than any of the former. In this, the Angles of Pofition, or the Bearings of the feveral Places, with re- gard to one another, are determin'd by proper Inftru- ments, and^ tranferr'd to Paper. This conftitutes an Art a-part, call'd Surveying. See Surveying.

The Ufe of Maps is very obvious from their Conftruclion. The Degrees of the Meridians and Parallels /hew the Longitudes and Latitudes of Places, and the Scale of Miles annex'd, their Diftance; the Situation of Places, with regard to each other, as well as to the Cardinal Points, appears by Infpection, the top of the Map being always the North, the bottom the South, the right-hand the Eaft, and the left the Weft ; unlefs the Compafs u- fually annex'd, fhew the contrary. See Mercator's Chart, where thefe Cafes are exemplify'd.

MAPPARIUS, an Officer among the Romans, who, in the public Games, as thofe of the Circus and the Gla- diators, gave the Signal for their beginning, by throwing an Handkerchief (Mappa) which he had before received from the Emperor, Conful, or other fupreme Officer then prefenr.

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