rectangular block ABCD, Fig. 195, and when the average crosssection of the vein is 6 feet, and a cubic foot of the vein matter in place weighs 150 lbs.

Ore stopes, or steps made in mine-workings by and for the extraction of ore, are generally very irregular, the representation here being an ideal one. Suppose the stope-faces to be 11 feet apart and 8 feet high, and that the inclined shaft has extracted 10 x 6 ft. of vein matter, and the levels 7 x 6 ft.

We see that the inclined shaft has exposed the vein for 440+115+115= 670 ft.; deducting say 15 ft. for inequality of surface, we should have a rectangular block 655 × (400 + 350) ×6 in width = 2947500 cubic feet of ore: to be deducted from this, we have

Stoped out on 1st level east, roughly estimated, 3400

Dividing by 134, the number of cu. ft. required for a ton, and we have 204852 tons of ore in sight.

Another method of calculation is as follows: The longest

drift east is 400 ft. and the shortest 100 ft. Assume the bounding line in this direction to be at a distance east of the shaft,

The longest drift west is 350 ft. and the shortest 100 ft.; take the bounding line in this direction at a distance

or the rectangular block abc F, Fig. 195. Calculate the orereserves, when the other data are the same as before.

This latter method is recommended by competent engineers as the fairest and most reliable for all parties concerned.

409. Fig. 197 shows a longitudinal section, and Fig. 198 a transverse section of a deposit mine with mill connections, the mill to be erected at the point A. From a consideration of the diagram, it is evident that the most convenient method for the transportation of the ore from the mine to the mill would be by a tunnel driven into the mountain, at the end of which is a bin, made in the solid rock and inclined to the tunnel at any convenient angle at which ore will slide into cars; the cars to be run into the tunnel on a track and directly under iron doors which are worked by rack and pinion. The bin is to connect with the ore-chamber by a chute inclined at an angle of 45°, as shown in the diagram.

The lower or mill tunnel should have a slope of 2 inches in 10 feet, so that the loaded cars would descend by the force of gravity, the last car in a train having a brake with which to regulate the speed. The chute should be 12 to 15 feet from the edge of the tunnel, to admit of constructing the inclined bin for the discharge of the ore into the cars. The point in the orechamber, at which it is desired to sink the chute, and the mouth

the lower tunnel being selected, drive a peg to the centre point

of the proposed tunnel floor and drive a nail in the peg, and repeat the operation at the point where the chute is to be sunk. Now make a careful traverse between these points; the direction of a line which will run from the mouth of the tunnel directly under the point selected for the chute can now be found, as explained in the section on traversing, and the course that will carry the tunnel 12 or 15 feet from the bottom of the chute may be determined. In driving the tunnel, holes should be drilled in the roof and wooden spuds driven in on which to hang plumb-bobs, the surveyor using great care to have the plumb-bobs suspended in the proper course as a guide to the miners. In starting the chute, a large wooden triangle should be made, one of the angles of which is the same as the angle of the chute, to be used by the miners as a guide, until sufficient depth is attained to hang, in the proper line, plumb-bobs, the points of which are on the required angle.

APPENDIX A.

THE SOLAR COMPASS.

(With some omissions, from Messrs. W. and L. E. Gurley's Manual of Engineering and Surveying Instruments, 24th Edition, 1883.)

This instrument, so ingeniously contrived for readily determining a true meridian or north and south line, was invented by William A. Burt, of Michigan, and patented by him in 1836.

It has since come into general use in the surveys of U. S. public lands, the principal lines of which are required to be run with reference to the true meridian.

The arrangement of its sockets and plates is similar to that of the Surveyors' Transit, except that the sight vanes are attached to the under plate or limb, and this revolves around the upper or vernier plate on which the solar apparatus is placed.

The limb is divided to half degrees, is figured in two rows, and reads by the two opposite verniers to single minutes.

The Solar Apparatus.-The Solar Apparatus is scen, Fig. 1, in the place of the needle, and in fact operates as its substitute in the field.

It consists mainly of three arcs of circles, by which can be set off the latitude of a place, the declination of the sun, and the hour of the day.

These arcs, designated in the cut by the letters a, b, and c, are therefore termed the latitude, the declination, and the hour arcs respectively.

The Latitude Arc, a, has its centre of motion in two pivots, one of which is seen at d, the other is concealed in the cut.

It is moved either up or down within a hollow arc, seen in the cut, by a tangent screw at f, and is securely fastened in any position by a clamp screw. The Latitude arc is graduated to quarter degrees, and reads by its vernier, e, to single minutes; it has a range of about thirty-five degrees, so as to be adjustable to the latitude of any place in the United States.

The Declination Arc, b, is also graduated to quarter degrees, and has a range of about twenty-eight degrees.

Its vernier, v, reading to single minutes, is fixed to a movable arm, h, having its centre of motion at the end of the declination arc at 9; the arm is moved over the surface of the declination arc, and its vernier set to any reading by turning the head of the tangent screw k. It is also securely clamped in any position by a screw, concealed in the engraving.

Solar Lenses and Lines.-At each end of the arm, h, is a rectangular block of brass, in which is set a small convex lens, having its focus on the