As local attraction frequently affects the needle at the bottom of the shaft, and so vitiates the surface survey that depends on its swinging the same at both points, a method of dispensing with the needle must be resorted to. This is the suspension of two plumb-lines from opposite sides of the shaft, the plummets hanging exactly over as much of the first underground line as the width of the shaft will allow.
The two plummet lines at surface then give the direction, and by trial the dial must be put exactly in line with them in order to prolong it correctly.
If there are two or more shafts sunk on the workings, it will be an easy matter to ascertain if the needle can be depended on for laying out any further surface work, as the underground survey connecting the shafts can be laid down on the surface, or the direct bearing and distance calculated, when its correctness is tested by the terminating point of the survey.
2. To find depth of shaft at any point, to cut a vein whose dip is known.
Rule:—Multiply natural tangent of angle of dip C ([Fig. 50].) by the distance from outcrop to proposed shaft AC. The result is the depth required, AB.
By Protractor and Scale.—Rule on paper a line AC of the required distance, then at C set off the angle of dip and draw AB at right angles to AC. Then scale off AB = depth of shafts.
Fig. 50.
3. Given Depth of Shaft and Angle of Dip to find where it outcrops.—Then AC = AB × natural tangent of angle ABC. Or by scale and protractor by inspection.