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

147.—Magnification of Reading.—With the trough compass it is very common to have some form of microscope for reading the needle more exactly. This may be done by a Ramsden eye-piece being placed directly over the needle, as is common in some German instruments. A much more convenient plan for certain instruments is to read the needle longitudinally. This is generally done by means of a transparent scale being placed across the end of the needle which is divided upon glass or horn. This may read to either the near or distant point of the needle. A very good form of needle reading is found in some French instruments. This is shown Fig. 33, where the compass is shown entirely enclosed in a tube C which protects it from dust. The needle N has a vertical point fixed upon its end at P which reads pretty closely to a scale of 10° divided upon glass at G by the eye-piece E. It has a lifter L pressed up by a milled-head screw M. Fig. 34 shows the graduated glass. This compass is attached beneath the limb of a theodolite, or in any other convenient position upon an instrument. The author has placed a compass constructed upon this principle in a telescope, in such a manner that the needle may be read with the eye-piece, so as to cut a line with a distant object coincident with the line cut by the principal telescope of the instrument at 0° of its graduation. This plan will be more fully explained with tacheometers, Chapter XII.

Fig. 33.—Needle with reader.

Fig. 34.—Scale at G.

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148.—The Prismatic Compass, shown Fig. 35, was invented by Charles August Schmalcalder in 1812. It is the most convenient portable instrument for reading magnetic bearings. Angles may be taken with great rapidity within about 15′ of arc by holding the instrument in the hand, or perhaps within 5′ if the instrument is of 4 to 6 inches diameter and placed on a stand. It is a most valuable instrument for filling in close details, such as may occur among buildings, trees, etc., after the principal points have been laid down from observations taken with the theodolite. The principles of the reflection of a prism were discussed, [art. 55, Fig. 3, p. 29.]

Fig. 35.—Ordinary prismatic compass.

Fig. 36.—Section of the same, but with mirror.

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