COMPASSES

Magnetic compass

The magnetic compass consists of a magnetized pointer (usually marked on the North end) free to align itself with Earth's magnetic field. A compass is any magnetically sensitive device capable of indicating the direction of the magnetic north of a planet's magnetosphere. The face of the compass generally highlights the cardinal points of north, south, east and west. Often, compasses are built as a stand alone sealedinstrument with a magnetized bar or needle turning freely upon a pivot, or moving in a fluid, thus able to point in a northerly and southerly direction.

The compass greatly improved the safety and efficiency of travel, especially ocean travel. A compass can be used to calculate heading, used with a sextant to calculate latitude, and with a marine chronometer to calculate longitude. It thus provides a much improved navigational capability that has only been recently supplanted by modern devices such as the Global Positioning System (GPS).

The compass was invented in ancient China around  and was used for navigation by the 11th century. The dry compass was invented in medieval Europe around .This was supplanted in the early 20th century by the liquid-filled magnetic compass.

Navigational compass

The invention of the navigational compass is generally credited by scholars to the ancient Chinese, who began using compasses first for feng-shui and then later for navigation sometime before the 11th century. The compass later appeared in Europe, India, and the Middle East due to the formation of the Mongol Empire by Genghis Khan which effectly eliminated all previous national barriers within the empire and allowed the safe transfer and transportation of both people and intellectual knowledge across the silk road from China to Europe and Midlle East.

Dry compass:

 The dry mariner's compass was invented in Europe around 1300. The dry mariner's compass consists of three elements: A freely pivoting needle on a pin enclosed in a little box with a glass cover and a wind rose, whereby "the wind rose or compass card is attached to a magnetized needle in such a manner that when placed on a pivot in a box fastened in line with the keel of the ship the card would turn as the ship changed direction, indicating always what course the ship was on". Later, compasses were often fitted into a gimbal mounting to reduce grounding of the needle or card when used on the pitching and rolling deck of a ship.

While pivoting needles in glass boxes had already been described by the French scholar Peter Peregrinus in 1269,and by the Egyptian scholar Ibn Simʿūn in 1300,traditionally Flavio Gioja (fl. 1302), an Italian pilot from Amalfi, has been credited with perfecting the sailor's compass by suspending its needle over a compass card, thus giving the compass its familiar appearance.Such a compass with the needle attached to a rotating card is also described in a commentary on Dante's Divine Comedyfrom 1380, while an earlier source refers to a portable compass in a box (1318),supporting the notion that the dry compass was known in Europe by then.

Bearing compass:

A bearing compass is a magnetic compass mounted in such a way that it allows the taking of bearings of objects by aligning them with the lubber line of the bearing compass.^[46] A surveyor's compass is a specialized compass made to accurately measure heading of landmarks and measure horizontal angles to help with map making. These were already in common use by the early 18th century and are described in the 1728 Cyclopaedia. The bearing compass was steadily reduced in size and weight to increase portability, resulting in a model that could be carried and operated in one hand. In 1885, a patent was granted for a hand compass fitted with a viewing prism and lens that enabled the user to accurately sight the heading of geographical landmarks, thus creating the prismatic compass.Another sighting method was by means of a reflective mirror. First patented in 1902, the Bézard compass consisted of a field compass with a mirror mounted above it.This arrangement enabled the user to align the compass with an objective while simultaneously viewing its bearing in the mirror.

Liquid compass:

The liquid compass is a design in which the magnetized needle or card is damped by fluid to protect against excessive swing or wobble, improving readability while reducing wear. A rudimentary working model of a liquid compass was introduced by Sir Edmund Halley at a meeting of the Royal Society in 1690.^[54] However, as early liquid compasses were fairly cumbersome and heavy, and subject to damage, their main advantage was aboard ship. Protected in a binnacle and normally gimbal-mounted, the liquid inside the compass housing effectively damped shock and vibration, while eliminating excessive swing and grounding of the card caused by the pitch and roll of the vessel. The first liquid mariner's compass believed practicable for limited use was patented by the Englishman Francis Crow in 1813.Liquid-damped marine compasses for ships and small boats were occasionally used by the British Royal Navy from the 1830s through 1860, but the standard Admiralty compass remained a dry-mount type.n the latter year, the American physicist and inventor Edward Samuel Ritchie patented a greatly improved liquid marine compass that was adopted in revised form for general use by the U.S. Navy, and later purchased by the Royal Navy as well.

Modern compasses

Modern compasses usually use a magnetized needle or dial inside a capsule completely filled with fluid (oil, kerosene, or alcohol is common). While older designs commonly incorporated a flexible diaphragm or airspace inside the capsule to allow for volume changes caused by temperature or altitude, modern liquid compasses utilize smaller housings and/or flexible materials for the capsule itself to accomplish the same result. The fluid dampens the movement of the needle and causes the needle to stabilize quickly rather than oscillate back and forth around magnetic north. North on the needle or dial, as well as other key points are often marked with phosphorescent, photoluminescent, or self-luminous materials  to enable the compass to be read at night or in poor light.

Thumb compass