NavList:

Given the diagrams in my last post, most people will have worked out how the Hughes bubble system in the MkIX series works.  However, I thought it worthwhile giving my explanation just in case anybody was still wondering.  As you thumb down towards you on the adjustment wheel, the diaphragm is stretched away from the chamber drawing liquid with it.  This liquid is replaced with liquid from the reservoir tube until all the liquid is used up and air/fluid vapour is drawn into the chamber to produce a large bubble.  Then you tilt the sextant about 40 degrees to the right until the bubble is against the tiny hole at half past ten (which you’re unlikely to see) in the edge of the chamber where you just saw the liquid and bubble squirt in from.  Then carefully back off the adjusting screw by thumbing upwards until the bubble begins to be sucked back into the reservoir.  As soon as the bubble shrinks to your preferred size flick the sextant back left again to trap the bubble in the chamber.  Finally, thumb the adjustment screw all the way up to relieve the diaphragm.  I know my old science master would say that there’s no such thing as sucking in, everything must be pushed from the far side, but you’ll get my drift.  Therefore, if refilling the system, it won’t do to fill the system completely full of liquid.  You’ve got to judge the level, so you’ll have enough air/vapour in the reservoir to do the pushing and to provide the bubble.  You might say "well you'd get bubbles eventually even if filled the reservoir completely", but you want them forming in the correct place, not like the ones in Champagne.

Michael Josefsson.  You appear to be getting on very well. You’ve reached the stage where I start blaming modern solder.

DaveP