It is better to temporarily mark all the points on the curve before driving in pegs, to see whether it works round all right; if not the process must be repeated until it does so.

It should be quite understood that the first offset is at right angles to the line A b, but the second and following ones are measured from c to c, d to D, and so on.

Setting out Curves from same Tangent.-Another method of setting out a curve by offsets is from the same tangent (Fig. 283), the offsets being all at right angles thereto. In this system the first Chord2 offset is found by the same rule as the preceding, viz., 2 Radius and the subsequent offsets are this result multiplied by the square of the number of points. Thus for a 20-feet curve :

Setting out Curves by means of Ordinates.-For small curves

this is possibly the most accurate method, consisting of setting up, upon a chord-line, ordinates, or lines at right angles thereto, to various points in the curve as illustrated in Fig. 284.

In Fig. 284, the letters refer as follows:

A

Fig. 284.

R = Radius.

X Distance of ordinate from centre of chord.

=

0 Length of ordinate.

I have not attempted to consider any of the other systems, although they look very well on paper, and many books are ornamented

with the abstruse calculations they involve, but "an ounce of practice is worth a pound of theory," and my experience induces me to say that, for mere approximation, the surveyor may use such of the methods by measurement only as I have described; but for accuracy and expedition in the field, and the assurance that you will be able to plot your curves upon your survey in the office, then the only reliable system is that advocated at the commencement of this chapter, and illustrated by Figs. 268 to 276.

CHAPTER XII.

OFFICE WORK.

NEXT to proficiency in all field operations, office work is of great importance. A man may be ever so clever a surveyor, and even renowned for his accuracy, but unless he can portray the results of his observations graphically, so that the least initiated can easily comprehend their meaning, his work will be deprived of a very considerable amount of merit. He may be an excellent draughtsman in some ways, yet fail utterly to give adequate expression to days or even weeks of patient labour, if he cannot in a minimised form give a true reproduction of his operations.

Necessity for System.-System is a very potent element in all branches of surveying, especially draughtsmanship. Those beautiful Ordnance plans, in various scales, are the result of accuracy in the field and methodical elaboration in the office. Take even the 1-inch map, and it seems to speak for itself; whilst the larger scales enable the authorities, by their perfect administration, to delineate the most minute features, of which these plans are faithful representations.

George Stephenson, in the early days of railway enterprise, was wont to express the opinion that a map or detailed drawing should be so executed as to enable either to be read "like a book;" and there is no reason whatever why a survey should not.

To this end, I wish to give a few preliminary hints which may be of service to the student.

Roughly Plot the Survey Lines.-1st. Roughly plot the chief lines of your survey to see what form it will take, so that you may arrange it symmetrically upon the paper upon which you intend to plot it.

Let the Paper be well seasoned.—2nd. Provide a piece of wellseasoned paper-Whatman's double-elephant, cold-pressed, is the best—and the paper should be mounted upon holland.

Draw a Scale on Paper before commencing.-3rd. Before commencing to plot your survey draw the scale upon the paper, so that you may apply your boxwood scales from time to time to ascertain whether the paper has been affected by temperature.

Boxwood Scales best.-4th. Boxwood scales are preferable to ivory.

Plot Survey North and South.-5th. Always plot your survey looking north, so that the top, bottom, left, and right respectively represent north, south, west, and east.

Paper Perfectly Flat.-6th. Keep your paper perfectly flat, and endeavour not to move it from the drawing table during the process of plotting.

Laying down the Survey Lines on Paper.-7th. Having made a rough plan of your principal lines, proceed to lay them down carefully upon the permanent paper, commencing with your principal base-lines.

Check Measurement.-8th. Measure each line from left to right (using a pricker) upon a faint pencil-line, and check back from right to left and test its accuracy.

Marking Stations.-9th. Mark round the puncture representing a station with a pencil-ring thus O, and opposite each station in faint pencil, with the distance, thus

O

Straight Edge.-10th. Having plotted your principal base and survey lines with a steel straight-edge (the longer the better), proceed to draw in these with a fine red line* (carmine or crimson lake), being specially careful that the lines are drawn accurately between the points only.

Never Plot from Pencil Lines.-11th. Under no circumstances plot your offsets or any detail lines from pencil chain-lines.

As to Plotting Long Lines.-12th. If the base or any other lines are longer than your straight-edge do not seek to produce the line hand-over-hand-wise, but take a silk thread and stretch it tightly between the extreme ends, and with a pricker (held perfectly vertical) make punctures at frequent intervening points, then you may apply the straight-edge, and be sure you have as true a line as is possible.

Plot all Survey Lines first. It is much better to plot all the survey-lines previous to commencing details, as any error, if detected, may be adjusted by re-measurement upon the ground, which might seriously affect the position of certain points of offset.

Plot each Day's Work as soon as possible.-Generally speaking it is better to plot each day's work at once. I do not say the same evening, for arduous duties in the field (often upon a very meagre meal) and a heavy feed on one's return from work are not con* A good surveyor need never be afraid of having the survey-lines upon his plan.

ducive to the patience, clearness of brain, or energy required for the purpose. On a large survey I recommend alternate days for field and office work, or using fine weather for the former, for say two or three days, and devoting wet days to office work.

Equipment of Office.-Now as to the equipment of an office. I differ entirely from those who argue that a surveyor who may have to take up temporary quarters at an hotel or inn, near the scene of his field operations, should plot his work under the very inconvenient circumstances often attending his sojourn. I am not speaking of a small survey, which may be plotted almost anywhere, and it is certainly preferable to do plotting in close proximity to the work rather than at a distance, in case of any mistakes in the chaining. But on a large survey it would be next to impossible to expect at an inn such facilities for plotting the work as are necessary, unless a room be specially engaged and fitted up for the purpose. This, however, must entirely depend upon circumstances, and no general rule can be laid down. Assuming, however, that arrangements of a satisfactory nature can be made, it is necessary for us to consider what are the necessary adjuncts of the office.

Drawing Tables.-1st. The drawing table is of great importance. It should be made of well-seasoned timber and free from all imper fections, such as knots, &c.;

it should be perfectly joined and clamped, and planed to an even surface. A convenient size is 8 ft. long by 4 ft. wide, and it should be supported upon a substantial under-framing with legs, not trestles. The edge

all round should have a bull-nose from 3 to 4 in. deep, and it is better to have a slot lengthwise on each side, so that the paper,

SECTION SHEWING SLOT.

Figs. 285 and 286.

if longer than the board, may pass through, and thus be protected from creasing during the process of plotting (see Figs. 285 and 286).

The paper should be held down by lead weights, 3" x 2" x 1" (weighing about 2 lbs.), covered with cloth or, preferably, washleather, and care should be observed in resting them, even so covered, by placing them on pieces of waste paper, in case of any defect in the covering, or dirt. I have already stated that a steel straight-edge should be provided, as long as possible (say 6 ft.), having a bevelled edge. This straight-edge should when done with