William Thomson with his developed level gauge leaning on a standard compass.
A Thomson level gauge is a level gauge that uses a prism. The circular frame is placed with a pin on the cap of the cover glass of a bearing compass or a daughter pointer of the gyrocompass. This allows ships or points to be identified by turning the device until the object is visible in the prism directly above the arrowhead on the chassis.
The bearing with a magnetic compass is called compass bearing KP, while a bearing with a gyrocompass gives a gyropeiling GP. The first must be corrected with the misconception to arrive at the true poll WP, the second with the total correction tc.
The advantage of the Thomson probe above the sight gauge is that it can also be used to probe celestial objects.
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