OT: refilling a spirit level bubble.

Given the need for absolute accuracy - otherwise the instrument is entirely useless - you may want to replace it entirely. Then there is re-installation and calibrating. I 'came up' on on optical transits... fun.

Peter Wieck Melrose Park, PA

Calibration is the easy bit, a 5km professional laser-levelled pair of fixed points locally about 150m apart. A few layers of nail varnish worked , for today, but I somehow doubt itwill be long-term reliable. Surveyed to a benchmark 100m away, to obtain the height, wrt Ordnance Survey datum. GPS , even dGPS is a waste of space in comparison to optical or laser levelling to good old benchmarks.

Unless you have a warm air layer in between refracting your line of sight.

I reckon a blob of silicon would complete the seal.

I might try that next time. The resultant bubble was a bit larger than I'd have liked, so next time leave to settle for a few hours, before topping up with spirit and then sealing over. I should say I've no access to a pro laser level and sensing staff but when there were anti-flood measures put in place locally, I got them to place a nail in the flood wall at a specific height relative to a distant specific level. Otherwise presuambly a matter of going to the coast , Abney placed near a calm sea edge and sighting the horizon.

Working near a very, very hot room and opening the doors to the cold, cold outside can cause that visual effect.

There is a rather famous case where tunnels were supposed to meet from each side of a mountain, but didn't. Analysis showed they'd relied on a laser level, and it had been bent by a warm air layer in the path.

Any surveyor can calibrate quickly with no instrumentation. Set level tripo d almost exactly between two objects approximately 200ft apart. Make a mark on each object (rotate 180deg back and forth). Said marks will be exactly at the same elevation. Move level (4-5ft) close to one mark, sight it, rota te to second mark (200ft away) and hope to read the same relationship. Adj ust bubble accordingly, and repeat entire procedure.

I think that imaging using electromagnetic radiation or waves has come quite a ways, too.