Looking to the right hand along the fence an object is there fixed for a station, turn the telescope to it, the angle reads 130° 30'; and, in order to have a check on all the angles at different parts of the of the survey, to prove both the chaining and the angles, take extra angles frequently from different stations to one particular object, either natural or one fixed purposely. In this instance a tree is the object, therefore an angle is taken at station (1) 38° 30′, and at the end of line (1) another angle is taken 70° 30'. Now the intersection of these two lines will fix that object on the plan, consequently when an angle is taken at station (3 or at station (26), or any other part of the survey, whatever the angle may be when it is plotted, the lines when drawn should intersect at the point or tree. Another way of proving the work in succession is, having the angles taken at each end of the line, the sum of those two gives the third, so that when those angles are plotted, measure the other angle by the protractor; it must read the correct angle if the previous work is correct. (See Problem 22.)

This process may be practised at different parts of the survey, and where there is no object put up a flag, and proceed as before.

Returning to 2), take the angles by the brook and road; also to the tree; continue in the same manner round a certain portion of the survey, taking the roads as a boundary as far as the case will admit, until finishing at the same point at which the survey commenced, as from 1 to 11), leaving pegs or marks on all the lines as stations for the interior fences.

Referring to the plan, observe that when lines from 1 to 11) and (12) to (13) are plotted, the only line that requires an angle measured by the instrument is (20); such lines may be taken with the box-sextant, after which every other line in that division is a proof line, and requires only the application of the scale.

The remaining portion of the survey is closed in the same

manner, and the numerous angles required on this side are properly checked by angles to the tree or other object in the

centre.

This system of surveying may fairly be termed a combination of the two, which cannot fail in producing an accurate survey.

One thing is to be observed, that when an error occurs either in taking an angle or in chaining, it is more readily discovered when the angles are taken from the meridian line than when taken by the other method.

In plotting, describe two or three circles by the outer edge of the protractor on different parts of the plan adjacent to the work, as shown by plan, and draw several meridian lines; it is then easier to draw the chain lines; and mark off as many stations on the circles as are in that locality, noting the angle and station.

Before attempting to plot the fences, let all the chain lines be first plotted, the number, length, and angle put against them; it will save much time, instead of having to refer frequently to the field-book.

The larger a protractor is, the better; the angles are more minutely pricked off. The wheel protractor is the most perfect instrument, having two verniers, and may read to seconds if required; but, unless strict attention be paid to the adjustment, by keeping the points truly in a line with the centre, the common protractor is far better. Incorrect plotting is just as bad as incorrect surveying.

For practice, plot this survey on a larger scale-the angles and chain lines only; then, to prove the survey, calculate by the figure A B C D E F all the triangles. Angles should also be taken from one station to another across the survey without chaining, as from C to D and from E to F, and, by joining all these lines, the whole survey is reduced to a figure of six sides. Note. The detailed portion of this survey is filled up as shown by the fieldbook, Plate 15.

Problem 30.

To survey the boundary and roads of an estate, Plate 23. In this example is shown a portion of the field-book, applying to this and the last plan.

This survey is intended to show only the boundaries and the roads, coinciding exactly with the observations made in the last example, proving at once how easily the whole estate may be filled up without further use of the instrument.

Distant objects should always be preferred in fixing the lines for chaining, particularly when using the theodolite, as angles may be taken from various parts of the survey to the same object, and be an excellent check to the whole of the survey.

Plot this survey to the same scale as the last for practice; compute the details of the former and the gross quantities of this, and compare the total quantities, which should agree within a few poles.

MINING SURVEYING.

Problem 31.

Latching or dialling a pit, Plate 24.

Latching or dialling is a term used by miners, when the survey of a mine is required of the course that has been excavated from one shaft of the mine to another.

This underground survey is then transferred to the estate plan by means of the shafts, which are accurately shown on the surface; otherwise it is working like a mole in the dark, and the course might run into another property.

Referring to the plan (which is an actual survey), it will be seen the lengths are taken by the chain or tape in links, and the angles taken by the needle, or magnetic meridian, with a circumferentor called by the miners a dial.

The legs of this instrument are made with a screw-joint in

the middle, and has a set of extra points to screw on when the mine is low, sometimes not more than a yard high.

The survey commences from the shaft No. 16 to No. 17. The chain lines and angles are all plotted from that point, in the same manner as described in Plate 20.

The circle shows the protractor with the meridian line through the centre, and all the angles marked thereon for plotting.

The field-book (if it may be so termed) is on the margin, containing only the numbers, lengths, and angles.

Great nicety is required in taking the angles and the lengths; the last point finishes at the centre of the shaft No. 17.

TOWN SURVEYING.

Problem 32.

Plate 25. The plan here represented is part of the town of Cheltenham, surveyed and published by the author.

In surveying parishes, it frequently occurs that large towns form a considerable portion; the streets are generally very irregular, preventing the possibility of continuing the course of triangulation.

When an opportunity presents itself to run a base line through the town from one side to the other, connecting itself with the general survey, it should be embraced, and fix on it stations for every street branching from it. Such a line as this will be a basis for the angles required to be taken by the theodolite. It is not required to take angles for every street, because, where two or more angles are taken on the same base, their position is fixed, and the lines running through the ends of them become fixed also, and many of them will close into the lines of the general survey outside the town.

As before described, there are two methods of taking angles by the theodolite: one, the angle measured from the magnetic

meridian; the other, the angle measured between two lines. There is a very great objection to the first, the uncertainty of the angle being correct because of the numerous attractions to the needle from iron railings, gas, water, and rain pipes, lampposts, &c., therefore the latter is more certain.

The four-pole chain is generally used, but when the survey is plotted to a large scale, and great accuracy required, the 100foot chain should be used; the offsets would then be taken with the tape in feet and inches, instead of links.

When flag poles cannot be fixed, take any object at a distance, as a lamp-post, corner of a building, &c.

Many station points are frequently referred to, either for taking angles or for starting fresh lines, and require to be found very accurately.

Therefore measure from each angle of the buildings nearest to it in feet and inches, as shown at C and B, Fig. 2; their intersections will give the true point.

The angles of buildings also require to be very minutely fixed; measuring an offset at right angles from the chain is not sufficiently accurate, therefore with the tape measure two distances from the chain, intersecting each other at the point required, as at 25 feet on the chain it is 37 feet to a, and at 50 feet on the chain it is 38 feet 3 inches to a, forming a small triangle; the same is done on the other side to b; the next angle is at c, and on the opposite side at d; the fronts of the buildings are straight from 6 to c, and from a to d; and so proceed on throughout, taking offsets in this manner only at the angles and for the stations. The subdivisions between each angle or corner of streets, &c., are all measured afterwards; the chief thing at first is to get all the lines and angles measured and plotted.

The line AH is directed to a lamp-post, or it might be extended to a line on the general survey, as at H; a station is left at C for lines D and E, to take up the opposite streets; also another station at B, for the two roads G and F. This line