Method of traversing with the Compass and Semicircle.

72. In running a subterranean traverse with the magnetic needle, a form of compass is used, called the Miner's Compass. It consists of a compass-box like that of the Surveyor's Compass, except that the graduation extends from 0° around, by the left, to 360°. This box is weighted at the bottom, and mounted on a universal joint, like the Mariner's Compass. The ring that supports the universal joint is provided with hooks for suspending it from a wire stretched along the gallery. When so suspended, the box assumes a horizontal position, the diameter through the 0 point of the graduated circle falling immediately under the wire; if the compass be so suspended that the end of this diameter points backward, and the 180° end in the direction the traverse is being run, the reading at the north end of the needle is the angular distance from the north point, around by the east to the direction of the course: this angla is called the Azimuth of the Course.

Miner's Semicircle.

73. The slope of the Course is measured by the Miner's Semicircle. This is a graduated semicircle, with hooks, for suspending it from a stretched wire, and a plumb-line attached at the centre of the circle. When suspended, the plane of the semicircle is vertical, and its diameter is parallel to the wire; the 0 point of the graduation is at the extremity of the radius which is perpendicular to the wire, and, as the divisions are numbered both ways, the reading of the plumb-line gives the slope. As it is impossible to stretch a wire so that it shall be perfectly straight, the slope measured at different points will not be the same; a fair result will be obtained by measuring the slope at each end of the wire, and taking the average as

74. To run a traverse, with the compass and semicircle, a copper wire is stretched from the place of beginning, say at the bottom of the shaft, to some convenient point of the gallery, and both ends made fast to iron hooks, driven into the walls of the gallery; the compass is suspended from this, at some point near its middle, and after it comes to rest, the azimuth of the course is read off and recorded; the semicircle is then suspended, first, near one end of the wire and then near the other, and the average reading is recorded, care being taken to note whether the slope is in elevation or depression; the length of the wire, from hook to hook, is then measured and entered in the field-book. If necessary, measurements are made to determine the cross section of the gallery, at any desired point of the course, and the results entered in the column of remarks. The wire is then detached from the hooks and carried forward along the gallery, one end being made fast at the extremity of the first course, and the other at some convenient point, generally on the opposite side of the gallery ; the same measurements are made as before and recorded under the proper headings, and so on, to the end of the traverse. The method of entering the field-notes, is shown in the following

Method of Traversing with the Theodolite.

75. In traversing with the theodolite, it becomes necessary to illuminate the cross hairs of the diaphragm. This is accomplished by a diagonal mirror, placed in front of the object-lens of the telescope. The mirror is supported by a stem projecting from a ring that surrounds the tube of the telescope, and has an opening in the prolongation of the tube, so as not to intercept the rays of light from the object. A lamp, placed on one side of the theodolite, illuminates the remainder of the mirror, and, if properly situated, a portion of its light is reflected into the tube of the telescope and thrown on the cross hairs. Sometimes the theodolite is furnished with three tripods exactly alike, upon each of which the instrument is mounted in turn, the other two serving to support the guiding lamps, which are so constructed that the flame of the lamp shall be as far above the top of the tripod as the axis of the vertical limb. Sometimes a single tripod is used, in which case the guiding lamps are placed on stands similar to the tripods, but with sliding pieces carrying the lamps, and fixed in position by clamp-screws. By this arrangement, the height of the lamp may be made equal to that of the theodolite. The lamps are protected by glass shades, of any color that may be desired.

To run a Traverse with a Theodolite having three Tripods. 76. Select a place for the first station, say at the foot of the shaft, and mark it permanently; set up a tripod over it, and place a station lamp on it: select a second station at some suitable point, generally at a turn in the gallery, set up a tripod over it, and on it put the theodolite: at the third station, still further along the gallery, set up the third tripod, and on it put a station lamp. The theodolite being levelled and the horizontal limb clamped, direct the telescope to the

the vertical limb will be the slope of the first course, in elevation, if the telescope point downward-in depression, if it point upward; then unclamp the vernier plate. and direct the telescope to the lamp at the third station, reading both limbs as before; the reading of the vertical limb will be the slope of the second course, in elevation, if the telescope point upwardin depression, if it point downward; subtract the first reading on the horizontal limb from the second (the latter increased by 360° if the 0 of the vernier has passed the 0 of the limb), and the difference will be the horizontal angle between the first and second courses.

The distance from the first to the second station is measured by chaining along the slope, the allignment being made by means of lamps, and the points corresponding to the ends of chains marked, by chalk-lines on the rock, or in some other convenient manner. The theodolite is then transferred to the third tripod, its place being taken by a station lamp, and the first tripod and lamp are carried along the gallery to mark the place of the fourth station; the same observations and measurements are made as before, and so on to the end of the traverse. The observations are recorded as shown in the following

When the bearing of the first course is known, the azimuth

of that, and of the following courses, may be found, and the

field-book, thus reduced to the form given in Art. 72. Sometimes, the first course is assumed as a line of departure, and the azimuths of the following courses are calculated with reference to it.

Modes of connecting with Surface Survey.

77. In order to determine the bearing of the first course, and also to be able to trace out the plan of the mine on the ground, it is often desirable to connect the underground traverse with the surface survey. There are two principal methods of making the connection.

A straight-edge, AB, is mounted on two trestles, and from it are suspended two plumb lines, E and F, as far apart as the breadth of the shaft will permit. To prevent agitation from currents of air, the bobs are permitted to dip into buckets of