This Method may be used in good Weather, if the Needle be well touched and plays freely; but if it be in windy Weather, or the Needle out of Order, it is better, after having taken the first Angle as before, and having removed your Inftrument to the fecond Station, and placed the Needle over the Meridian Line as before, to lay the Index on the laft drawn Line, and look backward thro' the Sights; if you then fee the Object in the first Station, the Table is fixed right, and the Needle is true, if not, turn the Table about, the Index lying on the laft Line, till thro' the Sights you fee the Object in the firft Station; and then fcrew it faft, and keeping the Edge of the Index to the fecond Station, direct your Sights to the next; draw a Line by the Edge of the Index, and lay off the next Line; and proceed fo thro' the whole without using the Needle, as you do with the Theodolite.

If the Sheet of Paper on the Table be not large enough to contain the Map of the Ground you Survey, you must put on a clean Sheet, when the other is full; and this is called Shifting of Paper, and is thus performed.

Plate VI. Fig. 8.

Let ABCD represent the Sheet of Paper on the Plane Table, upon which the Plot E, F, G, H, I, K, L, M, is to be drawn; let the firft Station be E, proceed as before from thence to F, and to G; then proceeding to H, you find there is not Room on your Paper for the Line GH: However, draw. as much of the Line GH, as the Paper can hold, or draw it to the Paper's Edge. Move your Infirument back to the firft Station E, and proceed the contrary Way to M, and to L; but in going

from

from thence to K, you again find your Sheet won't hold it; however, draw as much of the Line LK on the Sheet, as it can hold,

Take that Sheet off the Table, first obferving the Distance go of the Lines GH, and LK, by the Edge of the Table; take off that Sheet, and mark it with No. 1, to signify it to be the first taken off. Having then put on another Sheet, lay that Distance 00 on the contrary End of the Table, and fo proceed as before with the Refidue of the Survey, from o to H, to K, and thence to o; fo is your Survey compleat.

In the like Manner you may proceed to take off, and put on, as many Sheets as are convenient; and these may afterwards be joined together with Mouth Glue, or fine white Water very thin.

If the Index be fixed to the first Center, ufing the 360 Side, it will then ferve as a Theodolite, and when to the fecond Center, ufing the 180 Side, it will ferve as a Semicircle; by either of which you may furvey in rainy Weather, when you cannot have Paper on the Table.

Το

To take an Angle of Altitude by the Circumferentor, Theodolite, Semicircle, or Plane Table.

1. To take an Angle of Altitude by the Circumferentor.

L

ET the Glafs Lid be taken off, and let the Inftrument be turned on one Side, with the Stem of the Ball into the Notch of the Socket, fo that the Circle may be perpendicular to the Plane of the Horizon; let the Inftrument be placed in this Situation before the Object, fo that the Top thereof may be feen thro' the Sights: Let a Plummet be fufpended from the Center Pin, and the Object being then observed, the Complement of the Number of Degrees, comprehended between the Thread of the Plummet, and that Part of the Inftrument which is next your Eye, will give the Angle of Altitude required.

2. If an Angle of Altitude is to be taken by the Theodolite, or Semicircle, let a Thread be run thro' a Hole at the Center, and a Plummet be fufpended by it; turn the Inftrument on one Side, by the Help of the Ball and Notch in the Socket for that Purpofe, fo that the Thread may cut 90, having 360 Degrees next you: Screw it faft in that Pofition, and thro' the Sights cut the Top of the

Objects;

Objects; and the Degrees then cut by the End of the Index next you, are the Degrees of Elevation required. An Angle of Depreffion is taken the contrary Way.

3. By the Plane Table an Angle of Altitude is taken in the like Manner, by fufpending a Plummet from the Center thereof, having turned the Table on one Side, and fixed the Index to the Center by a Screw, fo as to move freely, let the Thread cut 90, look thro' the Sights as before, and you have the Angle of Elevation, and on the contrary that of Depreffion.

OF

OF THE

PROTRACTOR.

T

HE Protractor is a Semicircle annexed to a Scale, and is made of Brafs, Ivory, or Horn; its Diameter is generally about five or fix Inches.

The Semicircle contains three concentric Semicircles, at fuch Distances from each other, that the Spaces between them may contain Figures.

1

The outward Circle is numbered from the Right to the left Hand, with 10, 20, 30, &c. to 180 Degrees; the middlemoft the fame Way, from 180, to 360 Degrees; and the innermoft, from the upper Edge of the Scale both ways, from 10, 20, 30, &c. to 9c Degrees.

It is eafy to conceive that the Protractor, tho' a Semicircle, may be made to fupply the Place of a whole Circle; for if a Line be drawn, and the Center-hole of the Protractor be laid on any Point in that Line, the upper Edge of the Scale corresponding with that Line, the Divifions on the Edge of the Semicircle will run from o, to 180, from Right to Left: Again, if it be turned the other Way, or downwards, keeping the Centerhole thereof on the aforefaid Point in the Line, then the Divifions will run from 180 to 360,

and