541—Prismatic Mining Survey Compass.—This arrangement is designed by the author for very close workings. The entire depth of the instrument being only 4 inches, any reading may be taken from one point of view simultaneously with the observation. The 5-inch compass, Fig. 228, has a floating ring divided to half degrees, and the reading of this is reflected through a prism so that it appears directly under the fore sight, to be seen at the same time. The prism has a slight magnifying power, so that by estimation a bearing may be easily taken to ¼ degree or nearer. The principle of the compass is described [art. 148], the prism [art. 55]; but in this case the prism is raised and has a second lens under it, so that it forms a kind of prismatic Ramsden eye-piece. This elevation of the prism permits sighting under a certain amount of downward inclination, regulated by the height of the prism and the length of the back sight, as well as the upward inclination which is common to the use of prismatic compasses. The most important feature in this compass is the mode of lighting, which is effected by means of a large prism, Fig. 229 R, placed under the compass-box in a square tube, and a small movable lamp to throw light into it, Fig. 228 L. The floating ring, Fig. 229 C, is made of celluloid, quite transparent, so that the divisions upon it are clearly read through the small window in the cover of the compass-box. The fore sight W is jointed in two folds jj, so that it extends the distance of sights to about 10 inches apart in use, and yet folds away closely to the compass for portability when out of use. On the near sight a cut is made transversely to the slit. A second similar cut on the fore sight is made level with this to take levels roughly. About 20° are set off on each side of the cut on the fore sight, so that angles of altitude may be approximately taken—although the instrument is not well adapted to this. Two levels set at right angles to each other, to be used in setting up the instrument, are fixed under the compass-box. Weight of instrument, 4¼ lbs. without the tripod stand.
Fig. 229.—Section of prismatic mining compass.
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542.—Hanging Compass.—A very general method of underground surveying in mineral districts upon the Continent is by means of the hanging compass; this instrument is therefore generally found in catalogues of surveying instruments in France, Germany, and Italy. The original hanging compass was invented by Balthasar Rössler about 1660.[26] It appears to the author to be a valuable instrument for surveying in tortuous mineral veins where sighting is difficult. The measuring line upon which it is used is either a hempen or copper cord or a chain. The compass is hung upon the cord or chain, which may be stretched to any point out of sight, and the compass will then indicate the bearing of the line. In Germany two instruments are used simultaneously—the hanging compass for taking the bearing, and a clinometer, composed of a light brass semicircle graduated to degrees, with a small plummet for taking the inclination.
Fig. 230.—Stanley's hanging dial.
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543.—Hanging Dial.—Fig. 230 represents a modification of the hanging compass designed by the author, by which inclination may be taken simultaneously with bearing, if the dial can be suspended near the centre of the line or chain where the catenary curve is parallel with its points of support.
544.—In the construction of the instrument a circle of brass about 6 inches diameter, ½ inch wide, and 1/8 inch thick, has two arms extended to 12 inches at the upper part, on the end of each of which a hook is formed for hanging the instrument upon a cord or chain. Upon the lower part of the circle a fork-piece, with a bearing clipping the circle, is attached by two screws. The fork-piece is constructed to support two axes concentric to the vertical circle, in which the compass-box is suspended much above its centre of gravity, so that it falls by its own weight in use to a level position. Upon the edge of the compass-box an index is brought up nearly to the interior surface of the vertical circle, which reads into graduations upon this circle into degrees and half degrees.