PART V.

SECTION I.

It

OF LAND SURVEYING.

T is fuppofed that land-measuring first gave rife to geometry, which has fince been gradually riting to the height at which we at prefent view it. Since the divifion of common grounds has become fo frequent in England, furveying has been univerfally taught and practifed throughout the nation. I fhall here give a fhort account of two or three of the most useful instruments, before we enter upon the measurements themselves.

CHAPTER I.

The Defcription and Use of the most useful Inftruments for Surveying.

I. OF THE CHAIN.

22

Land is measured with a chain, called Gunter's chain, of 4 poles or 22 yards in length, which confifts of 100 equal links, each link being of a yard, or of a foot, or 7.92 inches long, that is nearly 8 inches or of a foot.

66

100

An acre of land, is equal to 10 fquare chains, that is, 10 chains in length and 1 chain in breadth. Or it is 220 X 22 or 4840 fquare yards. Or it is 40 X 4 or 160 fquare poles. Or it is 1000 X 100 or 100000 fquare links. These being all the fame quantity.

Alfo, an acre is divided into 4 parts called roods,

and

and a rood into 40 parts called perches, which are fquare poles, or the fquare of a pole of 5 yards long, or the fquare of of a chain, or of 25 links, which is 625 fquare links. So that the divifions of land measure will be thus:

The length of lines, measured with a chain, are set down in links as integers, every chain in length being 100 links; and not in chains and decimals. Therefore after the content is found, it will be in fquare links; then cut off five of the figures on the right-hand for decimals, and the reft will be acres. Thofe decimals

are then multiplied by 4 for roods, and the decimals of thefe again by 40 for perches.

EXAMPLES.

Suppose the length of a rectangular piece of ground be 792 links, and its breadth 385; to find the area in acres, roods, and perches.

2. OF THE PLAIN TABLE.

This inftrument confifts of a plain rectangular board of any convenient fize, the center of which, when used, is fixed by means of fcrews to a three-legged ftand, having a ball and focket, or joint, at the top, by means of which, when the legs are fixed on the ground, the table is inclined in any direction.

To the table belongs,

1. A frame of wood, made to fit round its edges, and to be taken off, for the convenience of putting a fheet of paper upon the table. The one fide of this frame is ufually divided into equal parts, for drawing lines across the table, parallel or perpendicular to the fides; and the other fide of the frame is divided into 360 degrees from a center which is in the middle of the table; by means of which the table is to be used as a theodolite, &c.

2. A needle and compafs fcrewed into the fide of the table, to point out the directions, and to be a check upon the fights.

3. An index, which is a brafs two-foot fcale, with either a small telescope, or open fights erected perpendicularly upon the ends. Thefe fights, and one edge of the index are in the fame plane, and that edge is called the fiducial edge of the index.

Before you use this inftrument, take a sheet of paper which will cover it, and wet it to make it expand; then spread it flat upon the table, preffing down the frame upon the edges, to ftretch it and keep it fixed there; and when the paper is become dry, it will by contracting again, ftretch itself smooth and flat from any cramps or unevennefs. Upon this paper is to be drawn the plan or form of the thing measured.

In ufing this inftrument, begin at any part of the ground you think the most proper, and make a point upon a convenient part of the paper or table, to reprefent that point of the ground; then fix in that point

one

one leg of the compaffes, or a fine steel pin, and apply to it the fiducial edge of the index, moving it round till through the fights you perceive fome remarkable object, as the corner of a field, &c, and from the station point draw a line with the point of the compaffes along the fiducial edge of the index; then set another object or corner, and draw its line; do the fame by another, and fo on till as many objects are fet as may be thought neceffary. Then meafure from your station towards as many of the objects as may be neceffary, and no more, taking the requifite offsets to corners or crooks in the hedges, &c, and lay the meafures down upon their refpective lines upon the table. Then, at any convenient place, measured to, fix the table in the fame pofition, and fet the objects which appear from thence, &c, as before; and thus continue till your work is finished, measuring fuch lines as are neceffary, and determining as many as you can by interfecting lines of direction drawn from different ftations.

And in thefe operations obferve the following particular cautions and directions.

1. Let the lines upon which you make stations be directed towards objects as far diftant as poffible; and when you have fet any fuch object, go round the table and look through the fights from the other end of the index, to fee if any other remarkable object lie directly oppofite; if there be not fuch an one, endeavour to find another forward object, such as shall have a remarkable backward oppofite one, and make ufe of it rather than the other; because the back object will be of ufe in fixing the table in the original pofition either when you have measured too near to the forward object, or when it may be hid from your fight at any neceffary ftation by intervening hedges, &c.

2. Let the faid lines upon which the stations are taken, be purfued as far as you conveniently can; for

that

that will be the means of preferving more accuracy in the work.

3. At each ftation it will be neceffary to prove the truth of it; that is, whether the table be ftraight in the line towards the object, and alfo whether the diftance be rightly measured and laid down on the paper. -To know if the table be fet down ftraight in the line; lay the index upon the table in any manner, and move the table about till through the fights you perceive either the fore or back object; then, without moving the table, go round it and look through the fights by the other end of the index, to fee if the other object can be perceived; if it be, the table is in the line; if not, it must be fhifted to one fide, according to your judgment, till through the fights both objects can be feen.-The aforefaid operation only informs you if the station be straight in the line; but to know if it be in the right part of the line, that is, if the distance has been rightly laid down; fix the table in the original pofition, by laying the index along the ftation line, and turning the table about till the fore and back objects appear through the fights, and then alfo will the needle point at the fame degree as at firft; then lay the index over the station point and any other point on the paper representing an object which can be feen from the ftation; and if the faid object appear ftraight through the fights, the ftation may be depended on as right; if not, the distance should be examined and corrected till the object can be so seen. And for this very useful purpose, it is advisable to have fome high object or two, which can be feen from the most part of the ground, accurately laid down on the paper from the beginning of the furvey, to ferve continually as proof objects.

When, from any ftation, the fore and back objects cannot both be seen, the agreement of the needle with one of them may be depended on for placing the table

ftraight