Disappearing air bubble in compass

[edelweiss]

While surviving a cold haulout and refit this NOV, DEC, JAN I noticed that a fairly large air bubble had apppeared in the glass of the binacle compass. The temperature had dropped to around 10F a couple of times. The air bubble looked to be about a 5ml volume ( it extended about 2 inches across. ). This was the first and only time I had ever seen it. Today (March) since it has been warmer and not so cold I checked it and the bubble is gone. I was delightfully surprised. The compass works perfectly as before.

Has anyone seen this happen? Any explanations? There may be a "expansion/contraction" diaphragm in the bottom of the Richey compass, but I was really surprised to see all that air!

[clonjers]

All you needed to know about Richie. Yes there is a diaphram.

http://www.ritchienavigation.com/service/manual.html