When on weather reporting ships Thomas. Had to repor.t on wave period and height of sea and swell. Cannot recollect any instrument for doing this. However for wind speed according to the Beaufort scale was supplied with a hand held wind anemonoter. Apart from other instruments supplied such as hydrometer and barograph.sea and swell heights were estimated by the observer. Would have been nice at the time to have an instrument for doing so. Who knows maybe some of us were unlucky. One thing I never encountered at sea was a bubble sextant used by aircraft. Thinking about it I suppose the bubble was to represent the sensible horizon, thus doing away with the dip correction. So then applying the parallax correction to get the altitude above the rational horizon. Any RAF navigators on site could clarify ?? Maybe if your distances off are incorrect could have something to do with air navigation. Maybe a wartime invention same as the US sight reduction tables for air navigation , which proved so useful to merchant ships also. May also be a stick for holding the tongue down by a medic when saying Aaaaaaggggghhhh and looking to see if you had a frog in the throat. Cheers.JWS. PS on the other hand could be to hang the key to the door of the lighthouse or even the outside khazi. JS.