NavList:

John Lance you wrote:  The triangular box section contains a bubble level exactly like the Admiral Gago Coutinho system albeit only one. I was hoping someone out there would know about the square section adjuster adjacent to the bubble slot.Does it alter the size of the bubble. In 2010 someone posted on this forum saying he owned a sextant with a 2 bubble system. I may have a punt on eBay but postage from the States is high.

John.  Are you sure there’s only a fore and aft spirit level?  The athwart ships one is less critical, because it isn’t part of the height sextant measuring system.  It’s used to prevent tilt error, so it can be quite short.  I would suggest that the square key is to adjust the level of one end of the fore and aft spirit level so that the bubble lies between its fiduciary marks when it also engages with the natural horizon.  Then, when you can’t see the natural horizon, you just keep the bubble within its fiduciary marks and bring the body down to the bubble.  I believe that sometimes a fiduciary mark on the horizon mirror is also included.  Bubble size adjustment tends to be a function of circular bubble chambers when the observed body has to be placed in the centre of the bubble, so the bubble can’t be too large, but it can’t be too small either or the bubble is slow to respond to movement.  DaveP