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Revised May 27 2021
The brass edge, originally designed to support the card, and throw the weight thereof as near the circumference as possible, is itself divided into degrees and halves; which may be easily estimated into smaller parts, if necessary. The divisions are determined by means of a catgut line stretched perpendicularly with the box, as near the brass edge as may be, that the parallax arising from a different position of the observer may be as little as possible.
There is also added an index at the top of the inner box, which may be fixed on or taken off at pleasure, and serves for all altitudes of the object. It consists of a bar, equal in length to the diameter of the inner box, each end being furnished with a perpendicular stile, with a slit parallel to the sides thereof; one of the slits is narrow, to which the eye is applied, and the other is wider, with a small catgut stretched up the middle of it, and from thence continued horizontally from the top of one stile to the top of the other. There is also a line drawn along the upper surface of the bar. These four, viz., the narrow slit, the horizontal catgut thread, the perpendicular one, and the line on the bar, are in the same plane, which disposes itself perpendicularly to the horizon when the inner-box is at rest and hangs free. This index does not move round, but is always placed on, so as to answer the same side of the box.
The sun's azimuth is known to be an angle contained between the meridian and the center of the sun. When this is required, and its rays are strong enough to cast a shadow, the box is turned about till the shadow of the horizontal thread, or, if the sun be too low, till that of the perpendicular thread, in one stile, or the slit through the other, falls upon the line in the index bar, or vibrates to an equal distance on each side of it, the box being gently touched if it vibrates too far; at the same time they observe the degree marked upon the brass edge of the catgut line. In counting the degree for the azimuth, or any other angle that is reckoned from the meridian, the outward circle of figures upon the brass edge is used; and the situation of the index, with respect to the card and needle, will always direct upon what quarter of the compass the object is placed. But if the sun does not shine out sufficiently strong, the eye is placed behind the narrow flit in one of the stiles, and the wooden box turned about till Some part of the horizontal or perpendicular thread appears to intersect the center of the sun, or vibrate to an equal distance on each side of it; smoked glass being used next the eye, if the sun's light is too strong. In this method another observer is necessary, to note the degree cut by the nonius, at the Same time the first gives notice that the thread appears to split the object.
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