Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

532.—Continental Forms of Miner's Dials.—On the Continent generally sights have been abandoned for miner's dials. The telescopes are usually of short form, with large object-glass and wide field of view. The telescope is generally placed eccentrically, which permits the instrument to be made of very low form. There is a certain amount of disadvantage in the eccentricity of the telescope, as angles cannot be taken direct from the centre of the instrument but this is compensated for in the plotting by making each station a small circle equal to the amount of the eccentricity of the instrument to scale, and setting off angles tangentially to this, which may be done with a little more trouble than that of plotting the angle from a point.

533.—French Miner's Compasses.—Fig. 222 shows the simpler form of this instrument. The needle is open and quite free from obstruction. The telescope is centred about level with the compass-box. The vertical axis has clamp and tangent adjustment. The transverse axis is set entirely by hand as with the plain dial. The instrument is set up level by its tribrach adjustment. The height with 5-inch needle in a level position, without tripod head, is about 5 inches; weight about 11 lbs. without the tripod table. The extremely squat form of the instrument permits its use in very close workings, with a short tripod, if the workings are fairly level. It is used also as a cheap form of surface surveying instrument, consequently it is not generally very carefully made. As a good instrument of the class it cannot compete with that to be next described.

Fig. 222.—French form of miner's dial.

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534.—It will be seen by Fig. 222 that the instrument has no direct connection with its stand or tripod. This is general with all French and German instruments, even with theodolites and surveying levels, it being the rule that the top of the tripod should form a kind of table upon which the instrument is set up. The table is almost uniformly made of wood, and is somewhat bulky and clumsy in construction, therefore not very well adapted to mining surveying, particularly in wet mines. Neither is the tribrach system of adjustment, unless it is supplemented by some form of ball and socket arrangement, or with adjustable stand. This subject will be further discussed in the description of superior instruments presently.

Fig. 223.—French miner's transit survey instrument.

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