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

Signalling with the Heliotrope.—A thin wooden bat D is moved over and off the outside front of the open disc aperture, following the rule of Morse signals, which will be presently described for the heliograph.

792.—Heliostat.—Is a smaller instrument than the heliotrope, in which the mirror or mirrors are moved by clockwork, so as to keep the sun's reflection in a uniform direction throughout the day. This instrument is delicate and not generally well adapted to field work.

Fig. 388.—Heliograph.

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793.—Heliograph.—This instrument is the invention of Sir H. C. Mance,[57] since improved by Major Macgregor, Colonel Bonham, and others. It is used for a military signalling apparatus, but it is also employed, on account of its portability in place of the heliotrope for surveying, where great precision by limiting the area of light reflection is not required. The construction of the instrument is shown in Fig. 388. B is the back of a plain circular mirror of 5 inches diameter, supported upon pivots on a fork frame J, the lower part of which forms a socket. The socket is furnished with a thumb-screw to secure the mirror and its frame when placed upon a cone projecting from the apparatus connected with the base plate formed on the top of the tripod head. The cone is erected upon a disc or wheel cut at its edge in teeth and centred upon the axis of the tripod head. The wheel is revolved by means of a pinion connected with a milled head A which moves the mirror and the entire apparatus above in horizontal revolution.

794.—The Sighting Arm L is attached to a collar fitting projected from the tripod head. This may be fixed in any horizontal direction by means of the tangent clamping screw C. The arm L has a supplementary extension by the piece Sj, which is jointed at the position of these letters and also by a socket fitting into the arm. The termination of the extension is a sighting point I formed of a thin blade of metal. The arm and its fittings permit the sighting point I to be set in any direction or elevation to follow the inclination of the land.

795.—The Sighting Vane is a piece of white metal upon which there is placed a black dot termed the sighting spot. A small circle, about 1/5 inch diameter, is left unsilvered in the centre of the mirror, which does not reflect the sun's rays. It therefore causes a small disc of shadow in the centre of the reflection of the mirror, termed the shadow spot. The shadow spot is made to appear upon the sighting spot when the instrument is adjusted to throw the sun's image upon a distant station.

796.—The Supplementary Mirror M is similar to that already described, centred also on pivots and placed in a forked frame. This is mounted on a cone S′ which fits into a socket at S, when the extension arm J is removed. This mirror is intended to receive the image of the sun when placed towards the back of the pointing of the instrument to throw the sun's image from the mirror M to B, to signal by double reflection, when the sun is at a forward angle to the distant station. The coincidence of reflection is taken with this mirror by the reflection of a piece of paper pasted on its centre of the same form as the index I.

797.—Telegraphing Apparatus, called technically flashing apparatus. This consists of a rod R hinged to the top of the mirror at its upper end and also to a lever which forms a Morse key at the lower end. The rod is formed of a screw of about half its length, which passes into a female screw tube so as to shorten or lengthen it as required to direct the reflection of the sun's rays by turning the milled head above R, which forms a part of the tube. The Morse key is hinged at J to the stem of the instrument, and is kept up to a fixed stop by means of a spring P extended by an arm from the stem of the instrument, so that pressure upon the disc F moves the key down to its stop P, and also tilts the mirror to throw its reflection off the observing station during the pressure. The flashing described by the jar of its action is liable to displace the mirrors. The use of the bat, shown at Fig. 387 D, is more certain for signalling words.