Britain, on the scale of 6 inches to the mile, contour lines are drawn at each 25 feet of height, with principal contour lines, determined with greater precision, at every 50 feet in the flatter parts of the country, and at every 100 feet in the hilly parts.

Mr. W. F. Howard advocates that colliery plans should exhibit contour lines at regular and frequent intervals. In this way the vertical throw of each fault, excluding the mere bending up or down of the adjacent strata, which has often a tendency to mislead, would be continuously shown though the fault should be rarely penetrated. Contour lines are generally shown on the plans of the anthracite mines of Pennsylvania.

CHAPTER XIV.

CONNECTION OF THE UNDERGROUND- AND SURFACE-SURVEYS.

Methods Employed.-A correct survey of the underground workings of a mine, and of the surface or royalty having been made, it is necessary to determine accurately the bearing of a line underground with a view to connect the two surveys. For this purpose the following methods have been employed:(1) By means of an adit-level or inclined shaft; (2) by means of two shafts; (3) by means of one shaft with two suspended plumb-lines; (4) by means of the transit-instrument; (5) by means of the transit-theodolite; (6) by means of the magneticneedle.

1. By Means of an Adit-level. When the mine is connected with the surface by means of an adit-level, the connection of the surveys is easily effected by continuing the underground traverse through the adit-level to the nearest side of the triangle of the surface-survey.

2. By Means of two Shafts.-If both the shafts are vertical, the connection of the underground- and surface-surveys is made

ут

B

by means of two plumb-lines, one suspended in each shaft. The points of suspension are joined to the surface-triangulation by means of careful measurements. In this way the length and bearing of the line joining the two plumblines may be calculated by means of rectangular co-ordiA traverse is then made underground from one plumbline to the other, and from the data thus obtained the length and bearing of the line joining the two plumb-lines is again calculated by means of co-ordinates.

nates.

Fig. 69.

Example. In two perpendicular shafts, plumb-lines are hung at the points A and B (Fig. 69). From the surface-triangulation it is found that the length of the line AB is 56-29 chains, and its bearing 118° 36'. In the mine, a traverse was made with the following results:

With the co-ordinates 7·20 chains N., and 55.81 chains E., the length and direction of the hypothenuse may be calculated from the formula: base2 + perpendicular2 = hypothenuse2, or tangent of angle of bearing

=

departure
latitude

and the distance latitude x

=

secant of angle of bearing. The hypothenuse in the above

traverse will then be found as follows:

From this angle-that is, the angle formed by the hypothenuse A B and the first line of the underground-survey A I, and from the bearing of the line A B determined at the surface (118° 36′), the bearing of the first station-line underground may be determined. In the above example, this is done by subtraction, 118° 36' 82° 38' 35° 58'. From this may be deduced the bearing of the other lines of the traverse. In the example, this is done by increasing the reduced meridian angles by 35° 58' in each case. With the aid of these bearings, the co-ordinates of the underground-traverse should be recalculated, and the results balanced.

=

For suspending the plummet, a thin wire of iron or brass is used. Hemp cords are useless for the purpose; because of their torsion and contracting when wet. They present a greater surface to the action of air-currents and water than thin wire, and do not admit of such precise sighting. The plummet weighs 5 to 8 lbs. It should not be hung on when the wire is let down the shaft in case of accident from the wire breaking. A smaller weight may be used when the wire is being let down, and at the bottom of the shaft it can easily be changed for the required weight. The plumb-line must, of course, hang perfectly free, without coming in contact with the sides of the shaft. To ensure this being the case, a lamp is slowly passed round the wire at the bottom of the shaft. If, in whatever position it is placed, the light can be seen from the top, the wire is clear.

The plumb-lines may be sighted without any difficulty in the surface-survey, as the upper part of each wire does not move. In the mine, however, the plumb-line has to be sighted at its lower end, which continues to vibrate like a pendulum. The motion may be reduced by allowing the plummet to dip into a bucket of water, and by shielding the wire from air-currents and falling water as far as possible. It is, however, impossible to stop the vibrations altogether.

To lessen the motion of the plumb-lines, Mr. H. D. Hoskold proposes the adoption of iron chains made from wire threesixteenths of an inch in diameter. The method would, however, be inapplicable in a shaft of considerable depth.

In sighting a plumb-line with the theodolite, it is best to follow it by means of the tangent-screw to the end of its vibration. There is then sufficient time to read the vernier before it reaches the other end of its course, as well as to intersect it in that position with the cross-wires. This operation is repeated several times, and the mean taken of all the results. When the arc is very small, the mean may be estimated, and the cross-wires set at that angle direct. The plumb-lines are rendered visible by

holding behind them a sheet of oiled paper illuminated by a lamp from behind. This method of sighting a plumb-line is very fatiguing, and necessitates great skill to read the vernier and direct the telescope to the next extremity of the course, in the comparatively short time in which the plummet completes its swing.

These difficulties have been overcome by Professor Schmidt,* of the Freiberg School of Mines. The plummets he uses are hung to thick wire (0·04 inch in diameter), and their weight is considerable, being as much as 50 lbs. They do not dip into water, but are allowed to swing freely. At a short distance above the bottom of the shaft, a horizontal finely-divided scale is placed perpendicular to the line of sight of the telescope. The swinging plumb-line is then observed with the telescope, and the successive extreme positions are read and noted, the plumb-line being purposely made to swing parallel to the plane of the scale. The latter is illuminated by means of an ordinary miner's lamp or candle.

From one or more series of double observations, the mean position of rest of the plummet on the scale is calculated, and for the subsequent survey the cross-wires of the telescope are made to coincide with that calculated point. The calculation of the position of rest is a very exact one. From two trials, one made at a depth of 557 feet under favourable conditions, the other at a depth of 1,722 feet under unfavourable conditions, Professor Schmidt found that the mean error of one series of observations. was± 0.12 inch, and the mean error of the result of a double series was ± 0·08 inch. Under unfavourable conditions the errors were 0.17 inch and 0·12 inch respectively.

In cases where it is required to connect the surface-survey with the underground survey at several levels at different heights. in the shaft, it is desirable to fix the plumb-line. For this purpose, Professor Schmidt† has invented a simple centering apparatus. On a perforated cast-iron plate, a prismatic centre-piece may be slid in two directions at right angles to one another by means of four centering screws. Above the latter are two scales at right angles. The iron plate is placed so that one pair of centering screws is in the line of sight of the theodolite-telescope, the other pair being in the line of sight of a second small telescope of low power. With this small telescope and with that of the theodolite, the swingings of the plumb-line are observed, and the position of rest calculated. The weight is then removed

*Saechs. Jahrbuch., 1882, p. 145.

Berg. H. Ztg., vol. xliii., 1884, p. 217.