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

Fig. 304.—Stanley's continuous arc box sextant.

Fig. 305.—Section of supplementary horizon arrangement.

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684.—Details of Spring Arrangement to the supplementary horizon glass are shown in Fig. 305 full size in section. The springs SS in Fig. 304 and S Fig. 305 form two points of support to the horizon glass, the silvered face of which is shown at A. A third point of contact is near D, placed in the centre of the end of the supporting plate for the horizon glass. When the screw R, which is placed in a loose fitting, is released, the springs bring the supporting plate tight up to D and hold the horizon glass firmly in an elevated position. When the screw R is tightened it brings this glass down. The horizon glass is adjusted over a rocking centre by the screws CC′. A screw and collar b prevent the loss of the screw R. By this arrangement the horizon glass is brought in or out of the field of view, in order to use the supplementary arc or for leaving it as a plain sextant.

Fig. 306.—Stanley's portable surveying sextant.

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685.—Open Surveying Sextants, similar to nautical sextants but generally smaller and of stronger construction, preceded the box sextant, and are still used to a limited extent upon the Continent, particularly with some form of supplementary arc, or arrangement to produce a large part of the reflecting circle. These forms are also occasionally revived by the opticians of our own country. The reason of this is easily seen. To the optician who lives in a town, moves on a level surface, and has comfortably warm hands, even in the winter, to hold and move the separate parts of an instrument, the open sextant appears the most perfect, as he can get at every part of it easily to clean and adjust. The surveyor takes another view of the subject. He is exposed in the open country to all weathers and all difficulties of movement over the land; therefore that form of instrument which is best protected and least liable to injury by a fall will be sure to be popular with him. It is upon these conditions the box sextant of some form is generally preferred.

A handy form of portable surveying sextant has been devised by the author and is shown at Fig. 306.