The fluid inside is mineral water. You can buy it at and marine store. Inside the compass underneath is a rubber seal that allows the fluid to expand when it''s warm. If it''s an old compass the rubber seal may be cracked. Or it can be loose. If it''s cracked you''ll need to get another.
The usual fluid inside a compass is a type of thin oil which has a low freezing point. This is not always readily available unless you have contact with a professional compass repair facility. I have used methyl alcohol (methanol, methyl hydrate) in my compasses for many years. It has a low freezing point and provides good dampening qualities for the card.