Initially “P” compasses were filled with a 90%:10% mixture of industrial alcohol (aka IMS 99) and distilled water; eventually it became 100% alcohol. Being industrial alcohol it is (and tastes) nothing like booze, and is, in fact, poisonous.
On the “spider” mechanism glass tubes are “glued” (with ordinary compass paint.) The 1940s versions had radium powder inside the tubes, which glows in the dark; post-war this was replaced by fluorescent powder, which needs a special light to make it glow; compasses with this modification should have an “M” added to their serial number, but that’s no guarantee. A good rule of thumb is to visually inspect the paint in the tubes, and on the cardinal points; if it’s brown, with black specks, it’s a fair bet that it’s radium-based, but don’t assume otherwise if the paint looks good. A Geiger counter is the only safe check.
Hidden from view, in the dome of the spider, is a pivot, which has a hardened chrome tip, but I wouldn’t like to hazard a guess as to how water might affect it.
The pivot fits into a depression in a sapphire “jewel,” which is held in triangular top to the “compass post,” and is free to rotate; it is usually considered (now) safest to store the compass upside-down, so that the pivot and jewel are not in constant contact, since the jewel can chip, and become useless.
Below the mechanism, in the base of the main bowl, is a flexible piece of corrugated metal, which takes up the difference in volume, caused by the effect, on the liquid, of height and temperature variations. During rebuilds, the liquid has to be de-aerated, to lessen the amount of this movement. As well as de-aerating the fluid, it’s normal practice to immerse the filler plug, at the same time, in some more liquid, and give it the same treatment, since the fibre seal is porous, and can trap air bubbles, which have a nasty habit of appearing at inopportune times.
The seal, under the bowl’s glass, is normally rubber, as found in “O” rings, and was 1/8″, but now 3mm; getting the seal the right size (usually, now, done by bevel cuts and super-glue,) and ensuring that it’s exactly in the right place, under the retaining bezel, can be a nightmare. Some compasses have been sealed with a form of two-part mastic, which is a real pain to remove.
The internal paint is a special compass type, which, once having contained the compass fluid, should never be allowed to dry out, since it’s prone to crack, and flake off, necessitating a repaint; this seems more prevalent with the white paint in landing compasses, less so with the black.
Hope that helps a little.
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