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About calibrating a cross-staff: I have this little scene in my mind of how 
a cross-staff maker might ply his trade.

The erstwhile navigator enters, saying that he wishes to buy. The proprietor 
fetches one of his stock out of the window. All these stock items have been 
made with the staff a bit extra-long.

"Stand exactly at this spot, the navigator is told, and with this instrument 
held by your eye, tell me the angle you make it between yonder horse-trough 
in the yard, and the baker's chimneytop across the alley."

and a moment later- "twenty two degrees and fortytwo minutes, you say? I'll 
soon fix that". He takes it into his workshop, and knowing that the angle 
should be 23� 15', can work out exactly how much to remove from the end of 
the staff to bring the reading up to scratch.

The mariner is asked to try again, and presuming that his reading is now the 
correct one, money changes hands, and both parties are happy, with the 
instrument now tailored to suit his own skull dimensions..

This is a scene that could be repeated over many years, as long as nobody 
shifted the horse-trough.

That's just imagination. I have no idea whether that's the way it was done, 
but it's possible.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 


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