intermediate stations, otherwise in an extensive survey the alphabet would soon be exhausted. A peg is driven into the ground at each station, and the chainage is always started from the peg, which denotes the station. Thus, in line 2 from station B to station C, the zero of the chain is placed at B when chaining the line B C. The position of the fence, twenty links back, can be most accurately determined by measuring this distance in the direction of the line B C, but this is done with the tape or with the chain, before measuring line 2. The total length of line 1 at station B is 352 links from station A. The details of keeping a field-book vary very much in the practice of individual surveyors, but in the Author's plan the total length at a station point is written in the field-book sideways, as shown, to distinguish it from the other chain measurements. The line is then continued on, to meet the fence at 365 links. In line 2 the measurement is continued on in a similar manner to meet the fence at 176 links.

The surveyor should endeavour to have as few pegs as possible near each other in filling in the details of a survey, a station from which several lines radiate being more correctly determined than where several single lines commence or terminate close to one another upon a base line. The exact distance of an intermediate station from the commencement of the base line may be best distinguished by enclosing the length entered in the field-book in an oval or oblong sketched in pencil, and placing a small circle with a dot in the centre upon the side, right or left, in which the branch line is to be chained. This is shown in the case of line 7, which is measured from 170, in the line 1, to the right hand of A B. It is not necessary to note the distance of all the poles employed for giving the direction of a base line. Only the chainage of those used for subsidiary triangles and side base lines need to be recorded.

The most usual form of field-book is that indicated in our diagram as pages 1, 2, and 3, with two central lines (see pages 47-49), generally ruled in blue, upon each page. of a field-book. As a rule it is not necessary to page a field book. The leaves of the book should be regarded as forming a continuous roll, divided into pages solely for con

D

venience of binding, and to be suitable for carrying in the pocket. The surveyor commences his entries at the bottom of a page, and works upwards as he proceeds along the base line, a method which a little reflection shows to be quite natural, and preferable to all others, as it places the survey-book in the same relative position as the base lines with respect to the surveyor, who advances towards the distant station. Thus the bottom of page 3 follows immediately the top of page 2 of the typical fieldbook which we have furnished. (See pages 47-49.)

To render the central column of the field-book intelligible, we will assume that a draughtsman could have no difficulty in plotting the irregular fence line which crosses the line marked M M, if perpendicular measurements are given, as shown, at distances say 10 links apart along the line M M. If this straight line, M M, be supposed to be widened out into a column, N N, sufficiently broad to write in the measurements taken along the line M M, and the offsets varying from 7 links to the right of 60, up to 29 links to the left of 210, be repeated, we have practically in the form N N a field-book, as indicated by page 2 in the diagram.

In keeping the field-book, therefore, it should always be remembered that the central column is virtually a representation of a chain line, and that the field-book should otherwise be as much as possible a fac-simile of the ground itself, with every post, hedge, house, pond, &c., placed on the pages of the book with regard to the central column, as they exist with regard to the chain line on the ground. It will be observed that the vertical lines for the offsets, measured from the line M M, or the column N N, are omitted in a field-book, and the distances only are figured, against the line indicating the fence, it being assumed that they are at right angles to the base line, unless otherwise sketched. As the central column is simply provided for the convenience of clearly recording the chain measurements taken along the base lines, and therefore practically indicates a single line, the fence line at 50, 160, and 290 links respectively, where it crosses the base line A B, should not be drawn across this column, but should be sketched, as shown, upon each side of the column, and the measure

ment at the intersection upon the chain stated in links by figures within the column, not in chains and links; thus the entry 956 would read, when plotting, 9 chains and 56 links, but the decimal point is never inserted in the fieldbook. When the 100 feet chain is used, the offsets are best booked in feet and fractions of a foot, not in feet and inches, thus 10 feet, 10 feet, 10 feet, 102 feet, 11 feet; since an entry II' o" might confuse with 1' 10" in plotting, and similarly in the case of other entries. Inattention in this particular causes much confusion in the relative position of offsets. The exercise of judgment is needed when taking the offsets, so as to select their correct positions, in order to avoid taking an unnecessary number of offsets.

As the chaining goes on, the surveyors mark and note the distances from the commencement, in each case, at which intermediate stations upon the main line would be suitable in the survey, also as in the case of A C and B D, the intersection of other base lines and the crossing of all fences and boundaries. Small triangles and other offsets not rectangular may best be recorded by sketching a diagram or the lines so measured and writing their lengths along them. Tie lines, instead of being measured exclusively for such purposes, should, where possible, be arranged so as to take up such features in the ground as it would be necessary to represent upon the plan. At the same time, whatever distance will in any way help to confirm the accuracy of base lines should be measured by the surveyor. Incorrect results, however slight, are a cause of much anxiety and perplexity. In case one tie line happens to be incorrectly measured or described, the surveyor should possess a proof of his work in the record of the length of other tie lines.

A small triangle with a dot in the centre is the mark adopted by the English Ordnance Surveyors for distinguishing their trigonometrical stations, and is recommended by the author for all main stations as shown in the field-book

given in the diagram, pages 47-49. Lettering A for reference is better than saying (as some books teach) from "end of line No....'; thus in the case of line 3 it is run from the end of line 2 and also from the end of line 5. The latter description would therefore be aml iguous.

CHAPTER VII.

POSITION OF BASE LINES.

IN open country the relative position of survey lines can be best fixed by chain measurements. In laying out base lines it must be remembered that the triangle is the only geometrical figure which is incapable of change of form without altering the lengths of the sides, and that when the position of the sides is proved by tie lines, it shows that the lengths of the base lines have been correctly measured. We shall allude to this more in detail when describing the means adopted for plotting a survey in a future chapter. A four-sided figure, like the frame of a slate with the slate out, can assume a variety of shapes without altering the lengths of the sides, but as soon as the corners are diagonally tied together, triangles are formed, and the position of the sides is fixed. In making a survey it is better to work from the whole to a part, than from a part to the whole, not adding field to field and acre to acre, but to embrace the whole survey in one grand system of triangulation. The longest base line should if possible extend throughout the whole length of the survey, and the exterior boundary be measured by offsets from base lines, which should be connected by triangulation to the longest line. The interior details can then be obtained by the formation of subsidiary triangles, and the measurements taken are recorded in a manner described in the previous chapter under the head of Field Book.

Experience alone can suggest the best position for the base lines, the angles between them being neither too obtuse nor too acute, but as equilateral as possible, as otherwise a very trivial error in the plotting of any one of the sides will materially alter the whole figure through the intersection being not well determined, and consequently