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Gary gave an outline of use of the gunner's quadrant and added:
"You can do the same kind of test with your level, place it on your
artificial horizon mirror, level it in one direction and then swap it
end for end and the bubble should recenter. However, if it doesn't,
you will not be able to determine the size of the error since you
don't have any way to measure it without additional equipment. "


It might be helpful to explain how to correct a level that is out of
adjustment. On any(flat) plane, there is at least one line that is
horizontal.
1)Put your level on the plane and rotate it until the bubble is in the
centre of the run.
2)Lay a heavy straight edge alongside it in contact and reverse the
level end for end. If the bubble is no longer central, halve the error
using the adjustment provided on the level.
3)GOTO 1)and repeat until the bubble stays central when reversed.

Your level is now in adjustment and the straight edge lies along a
line that is horizontal.

It may be that the level is so far out of adjustment that the bubble
goes off the scale on reversal and it may take many tries to get the
bubble somewhere on scale. In this case, you can save time by finding
two lines on your plane that give the same bubble reading. Bisect the
angle formed by the two lines to find the approximately horizontal
line to "bring in" the bubble.

A level that is in adjustment will of course give the same reading on
reversal on a line that is not horizontal(provided that the inside of
the vial is correctly ground to a segment of a circle and not some
other shape).

Bill



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