Definition 1 of 2
Plain English Meaning
A small, flexible bag with folded sides — like a tiny accordion — that can squeeze in or stretch out as the liquid around it warms up or cools down. Inside the compass, it quietly takes up the slack so the fluid stays full and the compass keeps working smoothly across temperature changes.
Full Definition
A flexible, accordion-style sealed chamber that expands and contracts in response to pressure or temperature changes. In a magnetic compass, a small bellows is used as a temperature-compensating element that adjusts the volume of the compass fluid as it expands or contracts with temperature, preventing bubbles from forming and keeping the float assembly properly supported.
Context Anchor
Seen in discussions of magnetic compass construction, especially how a wet compass handles temperature changes without leaking.
Derivation
From Old English 'belg', meaning bag or pouch — the same as 'belly'. The traditional blacksmith's bellows is a folded leather bag that pumps . In instruments, the same accordion shape lets a sealed chamber change without leaking.