Surveying
In the prismatic compass

the line of the sight does not move with the box
the magnetic needle moves with the box
the graduated circle is fixed to the box and the magnetic needle always remains in the N-S direction
the magnetic needle and graduated circle do not move with the box

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Surveying
The error due to eccentricity of inner and outer axes can be eliminated by

taking mean of several readings distributed over different portions of the graduated circle
reading both verniers and taking the mean of the two
taking both face observations and taking the mean of the two
double sighting

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