Page:Cyclopaedia, Chambers - Volume 1.djvu/356

 C H A

( 202 )

C H A

ble of Longitudes and Latitudes, in the fame manner as is laid down under Map. •

Hence, i. the Latitude and Longitude of a Ship being given, her Place is eatfly exhibited in the Chart, z. The Places F and G, to and from which the Ship fails being given in a Map, the right Line P G, drawn from the one to the other, makes, with the Meridian AB, an Angle AFG, enual to the Inclination of the Rhumb : And fance the Parts Ft, I 2, 2 G, intercepted between equidlftant Parallels, are equal ; and the Inclination of the right Line P G to all the Meridians or right Lines' parallel to A B, is the fame: the right Line F G truly reprefents the Rhumb. After the fame manner it may be fhewn that they exhi- bit the Latin Mecodynamicum, or Miles of Longitude truly.

Coroll "Plain Charts, therefore, may be usd to very good purpofe in directing a Ship ; provided care be taken there efcape no Error in the Diftance of the Places F and G. ConfiruUion of a Scale to correct the Errors of the Dis- tances in Tlain Charts.

1. Upon the right Line A B, (Tab. Fig. 10.) from the Map transfer five Degrees, and divide 'em into 300 equal Parts, or Geographical Miles. 2. On this defcribe a fraall Cir- cle ACB, to be divided into 90 equal Parts : If then tis dehYd to know how many Miles make five Degrees in the Parallel 50 ; in the Compares take the Interval 50, and transfer it on to the Diameter A B 5 the number of Miles requir'd will here be fhewn.

Coroll. If then a Ship fail on an Eaftern or Weftem Rhumb, out of the Equator ; the Miles anfwering to the Degrees of Longitude, will be found as in the preceding Article. If it fail on any Collateral Rhumb, (till the fail- ing is fuppos'd to be an Eaftern or Weftern Rhumb, in an intermediate Parallel, between the Parallel of the Place whence the Ship proceeds, and the Parallel of the Place at which /he arrives.

'Tis true, this Reduction, by an arithmetically mean Parallel, is not accurate ; yet is it frequently us'd in Prac- tice, as being accommodated to the Apprehenfions of the generality of' Mariners. In effect, it does not err any thing eonfidera'ble, if the whole Courfe be divided into Parts, whereof each does not exceed one Degree; whence it ap- pears advifable, not to take the Diameter of the Semicircle A B above one Degree, and to divide it almoft into Geo- graphical Miles.

For the Application of the "Plain Chart in failing. See 'Plain Sailing.

Reduced Chart, or Chart of ReduBion, ■ is that wherein the Meridians are reprefented by right Lines con- verging towards the Poles ; and the Parallels by right Lines parallel to one another, but unequal.

Thefe, therefore, it appears by their Conftruction, mult correct the Errors of the Plain Charts.

But fince the Parallels fhould cut the Meridians at right Angles; thefe Charts are defective, inafmuch as they ex- hibit the Parallels inclin'd to the Meridians.

Hence another kind of Reduced Charts has been invent- ed, wherein the Meridians are parallel, but the Degrees thereof unequal ; call'd Afercator's Charts.

Mercatorh Chart, is that wherein the Meridians and Parallels are reprefented by parallel right Lines ; but the Degrees on the Meridians are unequal ; itill increafing, as they approach the Pole, in the fame proportion as thole of thc'Parallels decreafe : by means whereof, the fame Pro- portion is preferv'd between 'em as on the Globe.

This Chart has its Name from that of the Author who firft propos'd it for ufe, and made the firft Chart of this Projection, N. Mercator : but neither was the Thought ori- ginally his own, as having been hinted by Ptolemy near 2.000 Years ago : Nor is the Perfection of it owing to him ; our Countryman Mr. Wright being the firft who demon- ftrated it, and fhew'd a ready way of conftructing it, by enlarging the Meridian Lir.e by the continual Addition of Secants-.

ConfiruBion of Mercator'; Chart.

1. Draw a right Line, and divide it into equal Parts, re- prefenting Degrees of Longitude either in the Equator, or in the Parallel wherein the Chart is to terminate. From the feveral Points of Divifion erect Perpendiculars to re- prefent Meridians ; fo as right Lines may cut 'cm all un- der the fame Angle, and therefore reprefent Rhumbs : Thus far as in the Plain Chan.

That the Degrees of the Meridians may have their juft proportion to thofe of the Parallels, the former are to be increas'd ; in regard the latter continue the fame, by rea- fon of the Parallelifm of the Meridians. See Degree.

With the Interval therefore of one Degree in the Equa- tor CD, (Tab. Hydrcg. Fig. 11.) defcribe the Quadrant CDE, and in D erect a Perpendicular DG; make the Arch D L equal to the Parallel of Latitude, and thro' L draw CG: this CG will be the cnlarg'd Degree of the Meridian, to be transfci'r'd to the Meridian of the Chart : The reft as in Plain Charts.

In Practice, fuppofe it requir'd to draw a MercatGr'a Chart from the 40th Degree of North Latitude to the 50th, and from the <Sth Degree of Longitude to the 18th. Firft draw a right Line reprefenting the 40th Parallel of the Equator ; which divide into 12 equal Parts, for the 12 Degrees of Longitude the Chart is to contain. Then take a Line of equal Parts, on a Scale whereof loo Parts are equal to each of thefe Degrees of Longitude ; and at each extreme of the Line raife two Perpendiculars, to reprefent two parallel Meridians to be divided, by the continual Ad- dition of Secants, which are prov'd to increafe in the fame Proportion, as the Degrees of Longitude fhould decreafe. See Secant.

Thus, for the Diftance from 40 Deg. of Latitude, take 1 3 ii equal Parts, from the Scale which is the Secant of 40 Deg. 30 Min. For the Diftance from 41 Deg. to 42 Deg. take 1354 equal Parts, from the Scale which is the Secant of 41 Deg. 30 Min. and fo on to the laft Degree of your Chart, which will be 154 equal Parts, viz. the Se- cant of 49 Deg. 30 Min. and will give the Diftance from 49 Deg. of Latitude to 50 Deg. By this means, the Degrees of Latitude will be augmented, in the fame Proportion as the Degrees of Longitude on the Globe decreafe.

The Meridians being divided, add the Card, or Com- pafs ; chufing fome convenient Place near the Middle there- of : From this draw a Line parallel to the divided Meri- dians, which will be the North Rhumb ; and from this the other 3 1 Points of the Compafs are to be fet off. See Compass.

Laftly, lay down the Towns, Ports, Iflands, Coafts, £=fc. from a Table of Longitudes and Latitudes 5 and the Chart is compleat.

luMercator's Charts, the Scale changes as the Latitude is chang'd : If then, v.g. a Ship fails between the 40th and 50th Parallels of Latitude, the Degrees of the Meridians between thofe two Parallels, are to be the Scale for mea- furing the Ship's Way : Whence it follows, that tho the Degrees of Longitude be equal in extent on the Chart, yet they muft contain unequal Numbers of Miles, or Leagues ; and that they will decreafe as they approach nearer the Pole, becaufe meafur'd by a Magnitude conti- nually increafing.j

This Chart is demonftratively true ; tho to appearance falfe : it is found by Experience very accurate, and withal eafy of Application. In effect, it has all the Qualifications requir'd to render it of fervice in Navigation ; yet do the generality of Mariners decline the ufe of it, and rather chufe to keep the old erroneous Plain Chart.

For the Ufe o/Mercator'i Chart in failing. See Mer- cator's Sailing.

Globular Chart, is a Projection fo call'd, from the Con- formity it bears to the Globe it felf 3 lately propos'd to the World by MeflH Senex, Wilfon, and Harris ; where- in the Meridians are inclin'd ; the Parallels equidiftant both, and curvelinear ; and the Rhumbs real Spirals, as on the Surface of the Globe.

This Projection is yet in its Infancy 5 and the Secret thereof in a few Hands : We have but little to fay there- fore as to its Merits or Failings ; e'er long its Conftruction and Ufe will be made publick. In the mean time, we may be allow'd to expect great Matters from it, as it comes out under the Protection of his Majefty's Patent, and with the Recommendations of feveral able Navigators, and among others that of Dr. Halley ; and as it has already {food the Teft of a pretty fevere Inquifition. The Cavillings of Mr. Hafleden, who has thought fit to cenfure it e'er he knows what it is, will weigh but little ; and the lefs, as 'tis no longer a Secret what are the Motives that have inflam'd his Zeal. We JTiall only add, that the Projection is perfectly agreeable to Nature, and therefore eafily conceivable^ and that it has been found to anfwer very exactly, even in ve- ry large Diftances ; where its failure, if it have any, muft needs be moll confpicuous. See Globular Sailing.

Charts compos 'd by Rhumbs and 2) i fiances, are thofe wherein there are no Meridians or Parallels ; but all is. ef- fected by the Rhumbs, and the Scale of Miles.

Thefe are chiefly us'd by the French, efpecially in the Mediterranean.

They are patch'd up, without any great Art, from the Obfervations of the Mariners ; any regular Account there- fore how to make them would be needlefs. They are only us'd in fhort Voyages.

CHARTA primarily fignifies a fort of thick Paper, not unlike our Cap-Paper. See Paper; fee alio Charter.

Charta Emforetica, in Pharmacy, &c. a kind of Pa- per made very foft and porous, us'd to filtre withal. See Emporetica, Filter, &c.

Charta is alfo us'd in our antient Cuftoms for a Char- ter, or Deed in Writing. See Charter,

Charta Magna, the Great Charter, is an antient Inftru- ment, containing feveral Privileges and Liberties granted to the Church and State, by Edward the Confefibr ; toge- ther