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Under view.
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Fig. 328.
The sights are taken by means of an alidade moving along a slotted bar, which itself slides along a bar fixed at the side of the board.
The object aimed at is sighted in a mirror attached to the end of the alidade and aligned with a point at its other end, and the alidade is clamped into position by a thumb-screw on the slotted bar.
The advantages of this instrument are many and will only be fully realised when it is in practical use. No backsights are necessary. Sights can be taken with one hand. The operator is always in a comfortable position and the object aimed at is always immediately in front of him. The alidade remains in position by means of the clamp screw along the ray drawn till the object sighted at is reached and the distance known, which is merely marked off by means of the scale on the alidade. It is invaluable when sketching rivers from launches or canoes when backsights are often impossible. It is light and portable, being easily carried slung over the shoulder in a canvas case.
712.—The method of using is simple and ensures a great degree of accuracy with a minimum amount of time and trouble:—