Page images
PDF
EPUB

arrow at the tenth extension of the chain. In order to keep an account of the number of times which the arrows are thus exchanged, they should each tie a knot on a string, carried for that purpose, and which may be fastened to the button, or button-hole of the coat; they should also call out the number of those exchanges, that the surveyer may have a check on them.

It is very necessary that the chain bearers should proceed in a straight line; to this end, the second, and all the succeeding arrows, should always be so placed, that the next foregoing one may be in a line with it, the place measured from, and that to which you are advancing; it is a very good method to set up a staff at every ten chains, as well for the purpose of a guide to preserve the rectilinear direction, as to prevent mistakes.

All distances of offsets from the chain line to any boundary which are less than a chain, are most conveniently measured by the offset staff; the measure must always be obtained in a direction perpendicular to the chain.*

The several problems that may be solved by the chain alone, will be found in that part of the work, which treats of practical geometry on the ground.†

* Various tables of English and foreign measures, useful to surveyors, &c. are annexed to this work.

The best method of surveying by the chain, and now generally used by the more skilful surveyors, I judge, a sketch of here will be acceptable to many readers. It consists of forming the estates into triangles, and applying lines within them parallel and contiguous to every fence and line to be laid down, with offsets from these lines when necessary. The peculiar advantage of this method is, that, after three lines are measured and laid down, every other line proves itself upon application. Thus, if the triangle a b c be laid down, and the points d and e given in the sides, when the line de has been measured for the purpose of taking a fence con- b tiguous to it, it will prove itself when laid down, from the two extremities being given.

e

a

This method cannot be used in woods, where the principal lines could not be observed, or in surveying roads or very detached parts of estates; in such cases recourse must be had to the theodolite, or other angular instrument.

DESCRIPTION AND USE OF THE SURVEYING

QUAD

RANT, FOR ADJUSTING AND REGULATING THE

MEASURES OBTAINED BY THE CHAIN WHEN USED.

ON HILLY GROUND, INVENTED BY R. KING, SUR

VEYOR.

There are two circumstances to be considered in the measuring of lines in an inclined situation; the first regards the plotting, or laying down the measures on paper; the second, the area, or superficial content of the land. With respect to the first, it is evident that the oblique lines will be longer than the horizontal ones, or base; if, therefore, the plan be laid down according to such measures, all the other parts thereof would be thereby pushed out of their true situations; hence it becomes necessary to reduce the hypothenusal lines to horizontal, which is easily effected by Mr. King's quadrant.

With respect to the area, there is a difference among surveyors; some contending that it should be made according to the hypothenusal; others, according to the horizontal lines; but, as all have agreed to the necessity of the reduction for the first purpose, we need not enter minutely into their reasons here; for, even if we admit that in some cases more may be grown on the hypothenusal plain than the horizontal, even then the area should be given according to both suppositions as the hilly and uneven ground requires more labour in the working.

The quadrant A B, fig. 1, plate 14, is fitted to a wooden square, which slides upon an offset staff, and may be fixed at any height by means of the serew C, which draws in the diagonal of the staff, thus embracing the four sides, and keeping the limb of the square perpendicular to the staff; the staff should be pointed with iron to prevent wear; when the staff is fixed in the ground on the station line, the

OF THE INSTRUMENTS USED IN MEASURING STRAIGHT LINES.

OF THE CHAIN.

The length of a straight line must be found mechanically by the chain, previous to ascertaining any distance by trigonometry; on the exactness of this mensuration the truth of the operations will depend. The surveyor, therefore, cannot be too careful in guarding against, rectifying, or making allowances for every possible error; and the chain should be examined previous and subsequent to every operation.

For the chain, however useful and necessary, is not infallible; it is liable to many errors. 1. In itself. 2. In the method of using it. 3. In the uncertainty of pitching the arrows; so that the surveyor, who wishes to obtain an accurate survey, will depend as little as possible upon it, using it only where absolutely necessary as a basis, and then with every possible precaution.

If the chain be stretched too tight, the rings will give, the arrow incline, and the measured base will appear shorter than it really is; on the other hand, if it be not drawn sufficiently tight, the measure obtained will be too long. I have been informed by an accurate and very intelligent surveyor, that when the chain has been much used, he has generally found it necessary to shorten it every second or third day. Chains made of strong wire are preferred.

Gunter's chain is the measure universally adopted in this kingdom for the purpose of land surveying, being exceedingly well adapted for the mensuration of land, and affording very expeditious methods of casting up what is measured. It is sixty-six feet, or four poles in length, and is divided into 100 links,

each link with the rings between them is 7.92 inches long, every tenth link is pointed out by pieces of brass of different shapes, for the more readily counting of the odd links.

The English acre is 4840 square yards, and Gunter's chain is 22 yards in length, and divided into 100 links; and the square chain, or 22 multiplied by 22, gives 484, exactly the tenth part of an acre; and ten chains squared are equal to one acre; consequently, as the chain is divided into 100 links, every superficial chain contains 100 multiplied by 100, that is 10,000 square links; and 10 superficial chains, or one acre contains 100,000 square links.

If, therefore, the content of a field, cast up in square links, be divided by 100,000, or, (which is the same thing) if from the content we cut off the five last figures, the remaining figure towards the left hand gives the content in acres, and consequently the number of acres at first sight; the remaining decimal fraction, multiplied by 4, gives the roods, and the decimal part of this last product multiplied by 40, gives the poles or perches.

Thus, if a field contains 16.54321 square links, we see immediately that it contains 16 acres, 54321 multiplied by 4, gives 2.17284 or 2 roods and 17284 parts; these, multiplied by 40, produce 6,91360, or 6 poles, 91360 parts.

Directions for using the chain. Marks are first to be set up at the places whose distances are to be obtained; the place where you begin may be called your first station; and the station to which you measure, the second station. Two persons are to hold the chain, one at each end; the foremost, or chain leader, must be provided with nine arrows, one of which is to be put down perpendicularly at the end of the chain when stretched out, and to be afterwards taken up by the follower, by way of keeping an account of the number of chains. When the arrows have all been put down, the leader must wait till the follower brings him the arrows, then proceeding onwards as before, but without leaving an

02

arrow at the tenth extension of the chain. In order to keep an account of the number of times which the arrows are thus exchanged, they should each tie a knot on a string, carried for that purpose, and which may be fastened to the button, or button-hole of the coat; they should also call out the number of those exchanges, that the surveyor may have a check on them.

It is very necessary that the chain bearers should proceed in a straight line; to this end, the second, and all the succeeding arrows, should always be so placed, that the next foregoing one may be in a line with it, the place measured from, and that to which you are advancing; it is a very good method to set up a staff at every ten chains, as well for the purpose of a guide to preserve the rectilinear direction, as to prevent mistakes.

All distances of offsets from the chain line to any boundary which are less than a chain, are most conveniently measured by the offset staff; the measure must always be obtained in a direction perpendicular - to the chain.*

The several problems that may be solved by the chain alone, will be found in that part of the work, which treats of practical geometry on the ground.

* Various tables of English and foreign measures, useful to surreyors, &c. are annexed to this work.

The best method of surveying by the chain, and now generally used by the more skilful surveyors, I judge, a sketch of here will be acceptable to many readers. It consists of forming the estates into triangles, and applying lines within them parallel and contiguous to every fence and line to be laid down, with offsets from these lines when necessary. The peculiar advantage of this method is, that, after three lines are measured and laid down, every other line proves itself upon application. Thus, if the triangle a be be laid down, and the points d and e given in the sides, when the line de has been measured for the purpose of taking a fence contiguous to it, it will prove itself when laid down, from the two extremities being given.

e

a

This method cannot be used in woods, where the principal lines could not be observed, or in surveying roads or very detached parts of estates; in such cases recourse must be had to the theodolite, or other angular instrument.

« PreviousContinue »