West London Brass Compass in Wood Box. Marked on top, along with numbers that indicate the time of day using a sundial.

Green felt interior to box, anchor in a circle symbol on lid. 

Size: 5 3/4 x 5 1/2 x 2 1/2 in. 

A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with magnetic north. Other methods may be used, including gyroscopes, magnetometers, and GPS receivers, but in antiquity they were created using lodestones, naturally magnetized pieces of the mineral magnetite. Compasses often show angles in degrees: north corresponds to 0°, and the angles increase clockwise, so east is 90°, south is 180°, and west is 270°. These numbers allow the compass to show azimuths or bearings which are commonly stated in degrees. One of the earliest known references to lodestone’s magnetic properties was made by 6th Century BC Greek philosopher Thales of Miletus, whom the ancient Greeks credited with discovering lodestone’s attraction to iron and other lodestones. The name magnet may come from lodestones found in Magnesia, Anatolia. The earliest Chinese literary reference to magnetism occurs in the 4th Century BC Book of the Devil Valley Master (Guiguzi). Among the Four Great Inventions, the magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty (since c. 206 BC), and later adopted for navigation by the Song Dynasty during the 11th Century. The first usage of a compass recorded in Western Europe and the Islamic world occurred around 1190. Dry compasses began to appear around 1300 in Medieval Europe and the Islamic world. This was supplanted in the early 20th Century by the liquid-filled magnetic compass. In general, all these devices function as a pointer to “magnetic north,” the local magnetic meridian, because the magnetized needle at its heart aligns itself with the horizontal component of the Earth’s magnetic field. The magnetic field exerts a torque on the needle, pulling the North end or pole of the needle approximately toward the Earth’s North magnetic pole, and pulling the other toward the Earth’s South magnetic pole. The needle is mounted on a low-friction pivot point so it can turn easily. When the compass is held level, the needle turns until, after a few seconds to allow oscillations to die out, it settles into its equilibrium orientation. If local variation between magnetic north and true north is known, then direction of magnetic north also gives direction of true north. 

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