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The quantities in the reference book are to be arranged in the consecutive order of their reference numbers; and figures are invariably to be used for the reference; the use of letters being less clear in practice.

It is essential that the plan, when completed, should be such as that the admeasurement on the plan of the inclosures therein represented should correspond with the quantities assigned to them in the book of reference. The original plain working plan is always deemed the most valuable document, with all the lines of construction, names and reference figures shown upon them.

"TRAVERSING."

In the description of the parish survey, provision has been made for the determination of the true position of every object on the surface. It frequently happens that a surveyor may be called upon to make merely a survey of a road, canal, river, or boundary, without its being required to survey any of the objects on either side. This operation is performed by a process called "Traversing,” by which, with the assistance of the theodolite or other angular instrument, the surveyor is enabled with great expedition to perform the work required.

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Let A, B, C, D, &c., in the diagram, represent a road to be surveyed, at the starting point (A), selected at the side of the road so that the traffic on the road may interfere with the work; a staff, with a cross bar fixed at the usual height of the axis of the theodolite above the ground, is erected; and from A a straight line is measured in the direction in which the road is to be surveyed to a point B, selected at or near the first turn or bend, and at the side of the road, offsets being taken to the right and left, as may be required, to terminate the width

and boundary of the road: these measurements are entered as usual in the fieldbook. At B the instrument is adjusted and set to zero, the telescope having first been directed to the staff at A: the upper plate being then unclamped, the readings of one or more conspicuous objects in the neighbourhood are taken; and lastly, the telescope being directed upon a staff fixed by an assistant at the forward station C, chosen at the next bend in the road, the upper plate is

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firmly clamped, and the forward angle read off. Proceeding from B to C, the line B C is measured with accompanying offsets as before, and the theodolite adjusted over the station C. The ranging staff brought forward from A and fixed at B, serves as the object on which the telescope is to be directed for the back angle, by means of the movement of the lower limb, the upper plate still remaining clamped to the last forward reading. The staff at B being properly intersected by the cross wires, the lower plate is then firmly clamped, and the upper plate having been. unclamped, the same conspicuous objects are intersected as before, and the telescope is afterwards directed to the next forward station D,-when the same operation is repeated, to be afterwards continued throughout the entire length of the road to be traversed.

It will be observed that, by leaving the upper plate clamped for the back reading at the same angle as the pre

ceding forward reading, the readings of the horizontal limb, for the angles forward, indicate the direction of each station with reference to the first line on which the telescope was set, which may therefore be called the first meridian. This method of reading saves, in the subsequent plotting, the trouble of changing the position of the protractor at every angle.

The angles read at each station may be entered, at the corresponding station in the field-book, at the end of the measurements relating to the station line immediately preceding, as in the form (page 183). If the "forward" angles be, as in the diagram, written at right angles to the direction of the "forward" lines, the angles of elevation or depression may be noted under the forward angles, fractionwise.

In the example, the line A B is the first meridian to which all the angles are referred; but during the progress of the work other lines are selected as meridians, in order to facilitate the plotting. To constitute any one of the lines a meridian line, it is only necessary to fix the vernier at zero for the back angle, instead of retaining the preceding forward angle.

Pickets are generally driven at each station, to mark the precise spot, in case it should be necessary to refer to it again; and at the close of the day's work, if the survey is incomplete, angles should in addition be taken to several fixed points near the last station, in order that it may be identified with ease when the work is resumed.

The reason for taking angles to surrounding conspicuous objects, (if they be not so distant as to make the angles of intersection very acute, and thereby form ill-conditioned triangles,) is that they may serve as a check on the work; for the several bearings on the same object should, in the plotting, meet at a common point of intersection. However, from the dependence of each new station line for its

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direction upon those that precede it, errors will, except under favourable circumstances, be introduced into the work. The different parts of a traverse should therefore be referred to certain accurately fixed points, and any

errors thus discovered, may, if small, be corrected and allowed for in the plotting.

The surveyor, in plotting, marks off around the first station the bearings of the angles referred to the first meridian; and, taking from the field-book, in succession, the length of each station line connected with that first meridian, he transfers the direction from the central point by means of a parallel ruler. The same operation is repeated around each station at which a new meridian has been assumed. The station lines, when checked and found correct as to length and direction, are marked in faint red lines, and the road itself is plotted from the offsets given in the field-book.

Highly-finished circular protractors are made in metal with verniers attached, capable of reading to minutes and fractions of minutes; but these protractors are liable to be altered in position by the movement given to the arms bearing the verniers. For general use, circular pasteboard protractors, with the interior circle cut out, will be found more convenient;-these can be made by describing a circle with a radius of (say) 5 inches, and dividing the circumference to one-half or one-fourth of a degree, by measuring from an accurately-divided diagonal scale the lengths of the chords as given in the table of natural sines, and applied in the manner explained in page Quantities less than one-half or one-fourth of a degree may be estimated by the eye.

119.

In road or other traversing, it would scarcely be necessary to read to seconds, if the direction of the lines were to be plotted solely by reference to the protractor. But independently of the fact, that, after a little practice, no more time is required to read the seconds than the single minutes, it may be observed here, that when great accuracy is sought, the direction of the lines is calculated by the resolution of right-angled triangles, in which the angle

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