distinct image, and they are therefore omitted. Near the eyepiece, and situated between that and the object-glass, is a diaphragm which contains two hairs or webs bisecting each other at right angles, one vertical and the other horizontal. When taking horizontal angles the vertical web is turned until it apparently cuts the centre of the sight object, and for vertical angles the horizontal web is employed. A large spirit level is attached to the under side of the telescope, so

о

FIG. 92.

that the instrument may be adjusted to a greater degree of accuracy when taking vertical angles than the small spirit levels at the base of the dials admit of. The telescope is supported by two arms, rotating on an axis, which is the geometrical centre of the arc of the circle. To one of these arms is fitted a vernier for reading angles from the vertical arc. Fig. 93 represents an improved form of the Hedley Dial by Messrs. Davis & Son, which will be found to fulfil all ordinary requirements. The special features of this instrument are an outside vernier similar to a theodolite, an improved form of

arc for taking vertical angles, and the sights are interchangeable with a telescope for more accurate work. The advantage of the outside vernier is that the reading is more easily obtained, being in a more accessible position. The arc of the ordinary dial is replaced by a circular box 13 inch diameter, with a dial plate graduated to degrees and traversed by a hand.

Ꮎ

Ꮻ

B

FIG. 93.

The Theodolite.-The theodolite is essentially an improved dial consisting of almost the same arrangements, but with different screws for fine adjustment and reading. For loose needle surveying it is not so convenient as the dial, but for angling in important positions, especially on the surface where it can be manipulated easily, the theodolite is by far the better instrument.

There are various forms of theodolites, but they differ in detail only, the general construction of each being the same. Fig. 94 represents Hoskold's Miner's Theodolite. It consists of two horizontal plates (A), the lower one of which is graduated into degrees consecutively from 0 to 360, and the upper one is

the vernier plate.

The vernier plate may be rotated round the graduated plate, and the angle can be read off with the vernier. On the centre of the vernier plate is the compass box (B), the circle of which is also graduated for magnetic bearings. Above the horizontal plates is fitted a telescope (C), which is capable of moving on a horizontal axis, and at the side of which is fitted a vertical graduated circle (D) for reading vertical angles, the index for it is also provided with a vernier. Two levels (E and F) are fitted on the lower portion of the instrument at right angles to each other for adjusting, and another (G) is placed near the top of the instrument for the more accurate adjustment of the telescope when required for vertical angles. In some instruments this latter level is fitted to the top of the telescope, and is of rather larger size.

Beg

FIG. 94.

J

The verniers of both the vertical and horizontal graduated circles are provided with microscopes (H and I) for the more accurate reading of the same. To take a sight with the instrument, it is first levelled with the four screws (J J) joining the parallel plates, between which is the ball and socket joint on the lower part of the figure. The instrument is now turned until the telescope is approximately in the line of sight, and is clamped on the vertical axis above the parallel plate by a thumb screw (K); another screw (L) on the upper portion of the instrument clamping the telescope. The telescope can now be correctly adjusted to the line of sight by a slow motion or tangent screw (M) fitted below the horizontal plates. If the vertical angle is also required, the horizontal cross wire of the telescope must be made to cut the sight object by means of another slow motion screw (N) fitted near the telescope clamping screw. The instrument is now adjusted to the line of sight, and the vertical angle

may be read off. Should the angle which another line makes with this base line be required, then the position of the index of the vernier plate should be at 0° previous to the first adjustment, and the clamping screw (O), which holds it in this position, should be released in order to adjust the instrument to the second line of sight. The graduated horizontal plate is retained stationary, but the vernier plate, together with the telescope, etc., may be turned to the second line of sight. When approximately adjusted the horizontal plates are again clamped by the screw (O), and the more correct adjustment made with another slow motion screw (P). The angle which the vernier plate has passed through may then be read off by the vernier with the dial.

The manipulating screws of a theodolite may be divided into three sets: 1st, the clamping screw (L) and the slow motion (N), which adjusts the telescope; 2nd, similar screws (O and P) for adjusting the vernier plate in a relative position with the horizontal graduated plate when taking a sight; and 3rd, similar screws (K and M) for adjusting the whole of the instrument horizontally on the vertical axis above the ball and socket joint to a line of sight.

In some theodolites two verniers, 180° apart, that is in a straight line with the centre of the circle, are provided for the readings of both the horizontal and vertical angles, to test the construction of the instrument; and if the two verniers give slightly different readings the mean of the two is taken.

CHAPTER XI

SURVEYING WITH THE MAGNETIC NEEDLE

Ordinary Method. To make a survey underground it is necessary to have some fixed station to commence from or finish at, which is shown on the plan on which the survey is to be plotted. The usual procedure is to have two such stations, one to commence the survey from, and the other to finish at, in order that, when the survey is plotted, it will be apparent whether it is correct or not.

All mines are now provided with two shafts, a downcast and an upcast, and these are included in the surface survey and plotted on the plan. If the shafts are vertical, which is usually the case with coal-mining shafts, we have at once two points on the plan which correspond with two stations underground, and from one of these stations the survey commences. As the mine becomes extensively worked it would be impracticable to survey from the shaft every time, so stations or "dial marks" are left every survey, or alternate survey, for the succeeding ones. The station is left on one of the sights of the survey, and a note is put in the book that a mark has been left at this point. The station is denoted underground by putting a ring of whitewash or paint round the spot, if it is a mine with a good dry roof, or by driving a wooden plug a few inches in diameter into the floor or roof.

As an example of how a mine is surveyed, take the long wall workings shown by Fig. 95. The survey would commence at one of the shafts, say the downcast, and proceed round the workings, and if possible, tie in at the upcast. The measuring