Construction
This instrument clearly reflects the shape and features of the Davis quadrant. The significant differences are the change in the upper arc to a simple triangular frame and the addition of an index arm. The triangular frame at the top spans 60° as did the arc on the backstaff. The main graduated arc subtends 30° as in the backstaff. The 30° arc is graduated in degrees and sixths of a degree, that is, at ten minute intervals.
The sighting vane of the backstaff is replaced with a sight (called an eye vane) mounted on the end of the index arm.
The index arm includes a nonius to allow reading the large scale with ten divisions between the graduations on the scale. This provides the navigator with the ability to read the scale to the nearest minute of arc. The index arm has a spirit level to allow the navigator to ensure that the index is horizontal even when he cannot see the horizon.
The instrument has a horizon vane like a Davis quadrant, but Elton refers to it as the shield or ray vane. The shield is attached to the label. The label is an arm that extends from the centre of the arc to the outside of the upper triangle and can be set to one of the three positions in the triangle (in the diagram, it appears to bisect the triangle as it is set to the centre or 30° position). At the upper end of the label is a Flamsteed glass or lens.
The three set positions allow the instrument to read 0° to 30°, 30° to 60° or 60° to 90°. The lens projects an image of the sun rather than a shadow of the sun on the shield. This provides an image even when the sky is hazy or lightly overcast. In addition, at the mid-span of the label there is a mounting point for a lantern to be used during nocturnal observations.
There are two spirit levels on the shield. One, called the azimuth tube, ensures that the plane of the instrument is vertical. The other is perpendicular to the shield and will indicate when the plane of the shield is vertical and the label is horizontal.
Read more about this topic: Elton's Quadrant
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