Ship's compasses

Declination and deviation don't matter so much if you're on foot with a map or in a car; generally, there are other things you can use to help you find your way and it's hardly catastrophic if you take a wrong turn or two. On a ship, far from land and in bad weather (so you can't navigate by the sky), it's a whole different matter. Before technological advances like GPS and radar came along, people's lives depended on navigating accurately by compass alone. That's why ship's compasses (sometimes called mariner's compasses) were much more sophisticated than the ones people typically used on land. In a modern ship's compass, the compass card is attached to a float with a number of magnetic needles underneath it and spins freely inside a large glass bowl filled with a mixture of alcohol and water (to minimize friction and absorb vibrations from the moving ship). The whole thing is mounted on gimbals (pivots) in a stand called a binnacle so it stays horizontal even when the ship is pitching (moving up and down) and rolling (rocking from side to side) in the waves.