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level ground, and by the sides of rivers. It has a wheel of 84 feet, or half a pole in circumference, upon which the machine turns; and the distance measured, is pointed out by an index, which is moved round by clock-work.
Levels, with telescopic or other sights, are used to find the level between place and place, or how much one place is higher or lower than another. And in measuring any sloping or oblique line, either ascending or descending, a small pocket level is useful for showing how many links for each chain are to be deducted, to reduce the line to the true horizontal length.
An offset staff is a very useful and necessary instrument for measuring the offsets and other short distances. It is 10 links in length, being divided and marked at each of the 10 links.
Ten small arrows, or rods of iron or wood, are used to mark the end of every chain length in measuring lines. And sometimes pickets, or staves with flags, are set up as marks or objects of direction.
Various scales are also used in protracting and measuring on the plan or paper; such as plane scales, line of chords, protractor, compasses, reducing scale, parallel and perpendicular rules, &c. Of plane scales, there should be several sizes, as a chain in 1 inch, a chain in of an inch, a chain in an inch, &c. And of these, the best for use are those that are laid on the very edges of the ivory scale, to prick off distances by, without compasses.
5.—OF THE FIELD-BOOK.
In surveying with the plane table, a field-book is not used, as every thing is drawn on the table immediately when it is measured. But in surveying with the theodolite, or any other instrument, some sort of a field-book must be used, to write down in it a register or account of all that is done and occurs relative to the survey in hand.
This book every one contrives and rules as he thinks fittest for himself. The following is a specimen of a form which has been formerly used. It is ruled into 3 columns: the middle, or principal column, is for the stations, angles, bearings, distances measured, &c.; and those on the right and left are for the offsets on the right and left, which are set against their corresponding distances in the middle column; as also for such remarks as may occur, and may be proper to note in drawing the plan, &c.
Here is the first station, where the angle or bearing is 105° 25'. On the left, at 73 links in the distances or principal line, is an offset of 92; and at 610 an offset of 24 to a cross hedge. On the right, at 0, or the beginning, an offset 25 to the corner of the field; at 248 Brown's boundary hedge commences; at 610 an offset 35; and at 954, the end of the first line, the 0 denotes its terminating in the hedge. And so on for the other stations.
A line is drawn under the work, at the end of every station line, to prevent confusion.
29, a tree.
40, a style.
16, a spring.
20, a pond.
But some skilful surveyors now make use of a different method for the field book, namely beginning at the bottom of the page and writing upwards; by which they sketch a neat boundary on either hand, as they pass along; an example of which will be given further on, in the method of surveying a large
In smaller surveys and measurement, a good way of setting down the work, is, to draw by the eye, on a piece of paper, a figure resembling that which is to be measured; and so writing the dimensions, as they are found, against the corresponding parts of the figure. And this method may be practised to a considerable extent, even in the larger surveys.
THE PRACTICE OF SURVEYING.
This part contains the several works proper to be done in the field, or the ways of measuring by all the instruments, and in all situations.
To measure a line or distance.
To measure a line on the ground with the chain: Having provided a chain, with ten small arrows, or rods, to stick one into the ground, as a mark, at the end of every chain; two persons take hold of the chain, one at each end of it; and all the ten arrows are taken by one of them, who goes foremost, and is called the leader, the other being called the follower, for distinction's sake.
A picket, or station-staff, being set up in the direction of the line to be measured, if there do not appear some marks naturally in that direction; they measure straight towards it, the leader fixing down an arrow at the end of every chain, which the follower always takes up, til all the ten arrows are used. They are then all returned to the leader, to use over again. And thus the arrows are changed from the one to the other at every ten chains' length, till the whole line is finished; then the number of changes of the arrows shows the number of tens, to which the follower adds the arrows he holds in his hand, and the number of links of another chain over to the mark or end of the line. So, if there have been three changes of the arrows, and the follower hold six arrows, and the end of the line cut off 45 links more, the whole length of the line is set down in links thus, 3645.
When the ground is on a declivity, ascending or descending; at every chain length, lay the offset staff, or link-staff down in the slope of the chain, upon which lay the small pocket level, to show how many links or parts the slope line is longer than the true level one; then draw the chain forward so many links or parts, which reduces the line to the horizontal direction.
To take angles and bearings.
Let B and C be two objects, or two pickets set up. perpendicular, and let it be required to take their bearings, or the angle formed between them at any station A.
WITH THE PLAIN TABLE.
The table being covered with a paper, and fixed on its stand; plant it at the station A, and fix a fine pin, or a point of the compasses, in a proper point of the paper, to represent the point A: close by the side of this pin lay the fiducial edge of the index, and turn it about, still touching the pin, till one object B can be seen through the sights: then by the fiducial edge of the index draw a line; in the very same manner draw another line in the direction of the other object C. And it is done.
2.- -WITH THE THEODOLITE, &c.
Direct the fixed sights along one of the lines, as AB, by turning the instrument about till the mark B is seen through these sights; and there screw the instrument fast. Then turn the moveable index about, till through its sights you see the other mark C. Then the degrees cut by the index, upon the graduated limb or ring of the instrument, show the quantity of the angle.
3. WITH THE MAGNETIC NEEDLE AND COMPASS.
Turn the instrument, or compass so, that the north end of the needle poitt to the flower-de-luce. Then direct the sights to one mark, as B, and note the degrees cut by the needle. Next direct the sights to the other mark C, and note again the degrees cut by the needle. Then their sum or difference, as the case is, will give the quantity of the angle BAC.
4.-BY MEASUREMENT WITH THE CHAIN, &C.
Measure one chain length, or any other length, along both directions, as to B and C; then measure the distance B, C, and it is done. This is easily trans
ferred to paper, by making a triangle ABC with these three lengths, and then measuring the angle A.
To measure the offsets.
Ahiklmn being a crooked hedge, or river, &c.: From A measure in a straight direction along the side of it to B. And in measuring along this line AB, observe when you are opposite any bends or corners of the hedge, as at c, d, e, &c.; and from thence measure the perpendicular offsets ch, di, &c., with the offset-staff, if they are not very large, otherwise with the chain itself. And the work is done. The register, or field-book, may be as follows:
Having set up marks at the corners, which is to be done in all cases where there are not marks naturally; measure with the chain from A to P, where a perpendicular would fall from the angle C, and set up a mark at P, noting down the distance AP. Then complete the distance AB by measuring from P to B. Having set down this measure, return to P, and measure the perpendicular PC. And thus, having the base and perpendicular, the area from them is easily found. Or, having the place P of the perpendicular, the triangle is easily constructed.
Or, measure all the three sides with the chain, and note them down. From which the content is easily found, or the figure constructed.
2.-BY TAKING one or more of the ANGLES.
Measure two sides, AB, AC, and the angle A between them. Or measure one side AB, and the two adjacent angles A and B. From either of these ways the figure is easily planned; then by measuring the perpendicular CP on the plan, and multiplying it by half AB, you have the content.
To measure a four-sided field.
1.-BY THE CHAIN.
CAD 41 20
Measure along either of the diagonals, as AC; and either the two perpendi culars DE, BF, as in the last problem; or else the sides AB, BC, CD, DA. From either of which the figure may be planned and computed as before directed.
OTHERWISE BY THE CHAIN.
Measure on the longest side, the distances AP, AQ, AB; and the perpendi culars PC, QD.
2.-BY TAKING ONE OR MORE OF THE ANGLES.
Measure the diagonal AC (see the last fig. but one), and the angles DAB, CAD, ACD. Or, measure the four sides, and any one of the angles as ABC.
To survey any field by the chain only.
Having set up marks at the corners, where necessary, of the proposed field ABCDEFG, walk over the ground, and consider how it can best be divided in triangles and trapeziums; and measure them separately as in the last two problems. Thus, the following figure is divided into the two trapeziums ABCG, GDEF, and the triangle GCD. Then, in the first trapezium, beginning at A, measure the diagonal AC, and the two perpendiculars Gm, Bn. Then, the base GC, and the perpendicular Dq. Lastly, the diagonal DF, and the two perpendiculars pE, oG. All which measures write against the corresponding parts of a rough figure drawn to resemble the figure to be surveyed, or set them down in any other form you choose.