these two lines. The upper edges of the rectangular plate are bevelled, so as to diminish the shadow on the paper. The paper, on which the plan is to be drawn, is fastened to a horizontal table. The plotting-instrument is then placed on the paper, and turned until the marked end of the needle points to the north. A line is then drawn along the side of the plate which will represent the north line of the plan. A point for commencing the plotting is selected, and the instrument turned at that point until the needle points to the bearing of the first line. A line is then drawn along the side of the instrument, and the required distance measured with a scale.

This method presents the advantage of plotting the survey with the actual instrument used to make it, and consequently with the same degree of approximation. But the errors due to magnetic influences are not eliminated, as the conditions are not

Fig. 31.

the same as they were in the mine. In a drawing office, too, there are always iron objects that may affect the needle. At the French collieries of Decize in Nièvre, a drawing office has been built in which all the ironwork has been replaced by copper, for the purpose of employing this instrument without inconvenience.

Surveying with the Hanging-compass in the Presence of Iron.-Numerous attempts have been made to modify the construction of the hanging-compass in such a way that it can be used for surveying over iron. Perhaps the most successful is the adjustable hanging-compass (Fig. 31), invented by Mr. Penkert* of Beuthen. It is so arranged that it can be centred under the point of junction of two cords, and thus the bearing of the two lines can be taken from the same place. The instrument is manufactured by A. Ott, of Kempten, Bavaria.

* Berg. H. Ztg., vol. xxxix., 1880, p. 9.

CHAPTER VIII.

THE THEODOLITE.

Historical Sketch.-In mine-surveys, where extreme accuracy is required, the theodolite should be employed. There is, however, no occasion for it to be used exclusively, as the modern vernier-dial is a form of theodolite, which from its simplicity and compactness is better adapted for underground work than the theodolite itself, and proved by severe tests to give highly satisfactory results. For surface-surveying the dial is, however, decidedly inferior to the theodolite.

The employment of improved instruments for measuring angles underground in place of the compass, dates from the end of the last century. In 1798, H. C. W. Breithaupt, of Cassel, invented a mine-surveying instrument, resembling an astrolabium. This was essentially a theodolite. It had a graduated horizontal circle with verniers, a vertical arc, a sighting-tube, and_a compass. Two sets of legs were used with the instrument. In the same year, Professor Guiliani, of Klagenfurt, invented a mining theodolite, calling it a catageolabium. Mine-surveys, too, were made at the end of the last century by the Polish General, Komarzewsk, with a graphomètre souterrain which he invented. Since 1832, the theodolite has been used, more or less, in all mine-surveys, where great accuracy is required. Theodolites specially constructed for mining purposes are now made in great numbers by the continental and American instrument-makers. In Great Britain, however, the tendency has been to improve the construction of the circumferenter, making it more and more like the theodolite, so that with it results can be obtained as accurate as those made with a German mining theodolite of the same size.

Description of the Theodolite.-The theodolite is the most important, but at the same time the most complicated, instrument used by the mine-surveyor. In general outline, it may be described as a telescope mounted on a horizontal and a vertical axis, in such a way that the horizontal and vertical rotation of its optical axis may be measured.

E

There are many forms of theodolite, but there are certain essential parts common to all. The cost of a theodolite being considerable, the mine-surveyor is, as a rule, not in a position to have several of different sizes. In purchasing an instrument, therefore, he must select one which will fulfil all his requirements. It must be sufficiently large to give accurate results in triangulation, and at the same time it must be sufficiently portable to be used in the mine. In its construction, all metal must be avoided that will affect the magneticneedle. The horizontal circle should be 5 inches in diameter, divided into halfdegrees, and provided with verniers reading to single minutes.

D

K

ست

Fig. 32.

F

B

The various parts of the theodolite are shown in Fig. 32. The most important part is the horizontal circle, G, which has its edge bevelled and graduated, the degrees being numbered continuously round it towards the right up to 360°. At the centre of this circle is another circular plate, the vernier-plate F, capable of rotation independently of the horizontal circle. On the vernier-plate is engraved a line, the index line, passing exactly through the centre, the end-points, or indices, extending to the graduation of the horizontal with verniers, read by means sometimes provided with plates

circle. The indices are provided
of the microscopes g, g, which are
of ivory to reflect light on the scale.

The horizontal circle and

the vernier-plate together are sometimes termed the horizontal limb, in which case G is called the lower limb. The instrumentmakers' names are (F) plate, and (G) limb.

On the vernier-plate are two uprights or supports, D, D, which support the horizontal axis, C, of the vertical or altitude circle, E. The latter is provided with two indices with verniers at the opposite ends of a horizontal bar, read by the microscopes e, e. The telescope A B is fixed directly upon the horizontal axis.

The horizontal circle is screwed by a flange to a brass vertical axis, K, passing through the collar of a clamp, where it may be fixed or loosened by the clamp screw, k. Below the collar, the vertical axis works freely on a ball and socket joint at its lower end. The ball and socket is placed between the parallel plates, L, M, which are provided with four levelling screws, . The vernier-plate is provided with two spirit-levels, ff, and a longer spirit-level, c, is attached to the telescope. The whole instrument is supported on a strong tripod stand.

60

90 Co

The vertical circle is divided into four quadrants, the degrees in each of which are numbered from 0° to 90°, as shown in Fig. 33. In the old-fashioned theodolite the vertical limb is a semicircle. This is surmounted by an oblong flat piece of brass, the stage, to the ends of which are screwed the two forked rests called Y's, by which the bell-metal collars of the telescope are supported. Under the telescope is a long spirit-level. A theodolite of this kind is termed a plane theodolite, whilst one with a complete circle as vertical limb is termed a transit theodolite. The advantages presented by the latter form are the greater vertical sweep of the telescope and the greater accuracy of the readings of the vertical limb.

09

06

Fig. 33.

Connected with the horizontal circle and vernier-plate, there are two screws, H, I, one of which, H, is a clamping screw and the other a slow-motion or tangent screw. When H is loose, the two plates, G and F, can be moved independently, but when the screw H is tightened they can only be moved separately by means of the tangent screw I. Beneath the horizontal plates, there are two screws, k and i, one of which, k, is a clamping screw and the other a tangent screw. When the screw k is loose, the whole of the upper part of the theodolite above the screw can be rotated in any direction, in which case the horizontal circle moves upon the double conical axis upon which it rests. On tightening the screw k, the upper part can be moved only by means of the tangent screw i. Two screws refer to the vertical