608.—Prismatic Clinometer.—This instrument was originally devised by the author about 1860. The form of the instrument, Fig. 273, is that of a prismatic compass, [art. 155]. A similar metal or card and talc dial to that of the prismatic is used, but this is centred upon a transverse axis which is pointed at the ends to fit into hollow centres. This card is weighted on one side, so that when the sights are in a truly horizontal position the prism will show the zero of the card cutting the sight line. If the instrument be inclined upwards or downwards, the degrees of elevation or depression will be indicated by the card retaining its pendulous position. This is a very convenient instrument for use with the box sextant, and as it is only of about half an inch in thickness, and of the same diameter, it will pack conveniently in the case with that instrument—weight, 8 oz.
Fig. 273.—Stanley's prismatic clinometer.
Fig. 274.—Barker's clinometer.
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In Using this Form of Clinometer the prism is raised or lowered in its sliding fitting until the divisions of the card are sharply defined. Then in looking over the edge of the prism through the slot above it, the hair in the window of the back sight will appear to cut the divisions of the card; and the object seen in the distance, in front of the hair to which the instrument is directed, will appear coincident with the number of degrees of inclination indicated by the card.
This clinometer is sometimes fitted upon a prismatic compass, so that inclines may be read by the same prism and sight arrangement. This is, however, done more neatly by the arrangement next described, if the instrument is intended to be used with the prismatic compass only, and is not wanted separately for use with the chain.
609.—Barker's Combined Prismatic Compass and Clinometer,[36] Fig. 274.—The prismatic compass of this arrangement is that of Hutchison's form, [art. 155]. The clinometer is of the same kind as that just described, but this, instead of being a separate part of the instrument capable of detachment, remains permanent. To effect this arrangement the clinometer card is mounted over the compass card on a pin axis instead of centres. A part of the clinometer card is cut away so as to permit the compass card to be read beneath. This cut-away part is held by a stop to a position out of the field of the prism when the instrument is to be used as a prismatic compass. When the stop is released and the instrument is held with its face vertical, the pendulous clinometer card comes into view, and cuts by its reading through the prism the sight line, as before described for the prismatic clinometer. The prism is focussed to the upper or lower dial by a long, sliding fitting. It is used as the instrument last described.
Fig. 275.—Continental form of clinometer (Burnier).