What did the Egyptians, Romans et al use for horizontals. They could
have constructed a form of U-Tube but not a moveable/flexible one.

OK Pundits, what do you rekon?

Chris

Those of you with 1km. window sills should watch out!!

Seriously though thanks for the answers. I usually do do that with a
spirit level, turn it through 180 deg and see if the difference is the
same.

The towers of the Forth Road Bridge are different distances apart from
base to tip, can't remember what it is but it can be calculated. Also
driving across the Nullabor plain can be enlightning.

Chris.