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

will be sufficient to show the method of keeping the book, and the system to be adopted throughout in taking the dimensions.

When there are a sufficient number of lines measured, proceed then to take the angles. It will be found in course of the survey that angles will not be required to every line or street, for when one or two of the principal lines are plotted, many others will fall into the work as proof lines; for example, Fig. 1.

Much information will be gained by a careful study of the plan. The dotted lines represent the chain lines; the north part called the block plan, marked A, having only the fronts of the buildings or streets; the south part, marked B, represents the plan when finished with all its details.

On the line ab, there are ten stations determined on in the first line. Commencing again at c, take the angle a cd, on which there are two stations, gh; commence again at e, take the angle aef. Now, it is evident, if the two former angles and the lengths are measured correctly, that the line gf will be a proof line, and also the line ihk; the station at k will be a fixed point.

We now commence at ƒ and proceed to l, at which point it will be necessary to take the angle mlf, and that angle will prove the length of c d and n k.

Now measure by the chain the line 7m, passing through dn, that being the end of the second line; therefore the distance I to d must prove itself, as that point was before determined by the angle taken at c.

At n, on the line 7 m, leave a station, then will 7 m be a fixed line for a combination of the survey on the north part.

Now measure the line from n to k, on which leave stations at o and p; the lines n k and o g will be both proof lines.

So that in the great portion of this part of the survey three angles only are required, and every measured line afterwards proves itself.

The same principle is adopted on the south side, taking the longest and straightest streets to form a basis, and those lines

that are nearest to a right angle, as q to r, upon which numerous stations will be fixed.

Observe always to keep the work together as much as possible, so that it may be closed in, as shown by the lines e to s and s to t.

When the whole of the survey is completed, and the block plan plotted, proceed then with the details.

Prepare a book about the size of letter paper; copy off each block separately on an enlarged scale. With the tape, first measure the fronts of each house, passage, &c., separately, as thus (see Field-book): 24.3, 5.6, 20.0, and 29.3; add all these together, which should be equal to the measure of the whole length from b to c.

Then proceed with the depths of the houses and the back premises, taking diagonals from one angle to the other. A sketch of the premises having been previously made.

In many instances there are passages at the back, as at u v, which afford considerable assistance in measuring the details, particularly when straight, as at u; when they are crooked as at v, the prismatic compass will be found useful, as the angles must be taken by the needle.

In all cases take the supplement as well as the angle. (See Field-book.)

RAILROAD SURVEYING.

Problem 33.

Plate 26. To survey for a railroad.

When a survey has to be made for a railroad, it takes a long irregular tract of country from one important town to another, generally from 10 to 20 chains in width, in some cases more.

The course the intended line is proposed to be made is marked out by the engineer on the Ordnance or county maps.

The first and most important part in surveying for a line of railway is to determine and accurately pole out the base lines,

which should be as near as possible to the line described on the Ordnance map.

In chaining a base line the greatest attention and accuracy is required, as the only check to its length is when the levels are taken. Many bills in Parliament have been lost entirely through the inaccuracy of the plans and sections, involving not only the loss of a year, but the enormous amount of money required for the Parliamentary preparations.

Unfortunately these surveys are often got up in haste, and frequently under great opposition, all contributing to the many difficulties the surveyor has to contend against.

When a long line of country has to be surveyed a portion is allotted to each surveyor, as shown by the plans; each portion has to be connected at each end, requiring the greatest accuracy in fixing the base lines.

The most perfect method of connecting these lines, is by continuing them into each other's work, and fixing them by intersecting triangles.

Base lines that are connected by an angle taken with the theodolite are very ambiguous, from many causes; for instance, where the country is hilly there may be only a short distance from the instrument to the flag, and the line probably of considerable length, depending wholly on the angle taken by a sixinch radius; a very trifling error in reading the angle would throw that line greatly out of its real position. By the former method, if the lines are accurately poled out, the intersecting lines not only prove the correct position of each base, but they also serve to take up the details, and a great saving of time. Sometimes a base has been hung on by a fence only; such practice is highly censurable.

In a long line of railway it is impossible always to keep in the valley; a portion of the line will be rough and hilly; in such cases the theodolite is indispensable.

At the end of each base line the surveyor for that portion