Definition
A double-reflecting instrument with an arc of about 60 degrees, one sixth of a circle, that reads angles up to roughly 120 degrees through the doubling effect of two mirrors. The index arm carries the index mirror, while the fixed horizon mirror allows the reflected body and the direct horizon to be seen together. A micrometer drum and vernier permit reading to 0.2 minutes of arc, equivalent to about 0.2 nautical miles in the resulting position line.
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