describe an hour glass and explain the principle of making it.
Answers
Hourglass, a device for measuring time. In its usual form it consists of two cone-shaped or oval glass receptacles joined by a narrow neck. Sand or a liquid (such as water or mercury) in the uppermost section of a true hourglass will run through the neck into the lower section in exactly one hour. By turning the other end up, another hour may be marked, and the process may be continued indefinitely. When sand is used, the device is sometimes called a sandglass. A small sandglass, in which the sand passes from top to bottom in three minutes, is used for timing the boiling of eggs.
An early instrument working on the same principle was the clepsydra, or water clock.
ourglasses were an early dependable and accurate measure of time. The rate of flow of the sand is independent of the depth in the upper reservoir, and the instrument will not freeze in cold weather.[5] From the 15th century onwards, hourglasses were being used in a range of applications at sea, in the church, in industry, and in cookery.
During the voyage of Ferdinand Magellan around the globe, 18 hourglasses from Barcelona were in the ship's inventory, after the trip had been authorized by King Charles I of Spain.[12] It was the job of a ship's page to turn the hourglasses and thus provide the times for the ship's log. Noon was the reference time for navigation, which did not depend on the glass, as the sun would be at its zenith.[13] A number of sandglasses could be fixed in a common frame, each with a different operating time, e.g. as in a four-way Italian sandglass likely from the 17th century, in the collections of the Science Museum, in South Kensington, London, which could measure intervals of quarter, half, three-quarters, and one hour (and which were also used in churches, for priests and ministers to measure lengths of sermons).[14]