, a kind of water clock, or an hourglass serving to measure time by the fall of a certain quantity, commonly out of one vessel into another.— There have been also clepsydræ made with quicksilver; and the term is also used for hour-glasses of sand.

By this instrument the Egyptians measured their time and the course of the sun. Also Tycho Brahe, in modern times, made use of it to measure the motion of the stars, &c; and Dudley used the same contrivance in making all his maritime observations.

The use of Clepsydræ is very ancient. They were probably invented in Egypt under the Ptolemys; though some authors ascribe the invention of them to the Greeks, and others to the Romans. Pliny informs us, that Scipio Nasica, about 150 years before Christ, gave the first hint for the construction of them: and Pancirollus has particularly described them. According to his account, the clepsydra was a vessel made of glass, with a small hole in the bottom, edged with gold: in the upper part of this vessel a line was drawn, and marked with the 12 hours: the vessel was silled with water, and a cork with a pin fixed in it floated on the surface, pointing to the first hour; and as the water sunk in the vessel by issuing out of the small hole, the pin indicated the other hours as it descended.

Clepsydræ were chiefly used in the winter; as sundials served for the summer. They had however two great defects; the one, that the water ran out more or less easily, as the air was more or less dense; the other, that the water flowed more rapidly at the beginning, than towards the conclusion when its quantity and pressure were much decreased. Amontons has invented a clepsydra which, it is said, is free from both these inconveniences; and the same effect is produced by one described by Mr. Hamilton, in the Philos. Trans. vol. 44, pa. 171, or Abridg. vol. 10, pa. 248. Varignon too, in the Memoires de l'Acad. 1699, delivers a general geometrical method of making clepsydræ, or waterclocks, with any kind of vessels, and with any given orisices for the water to run through.

Vitruvius, in lib. 9 of his Achitecture, treats of these instruments; and Pliny in chap. 60, lib. 7, says that Scipio Nasica was the first who measured time at Rome by clepsydræ, or water-clocks. Gesner, in his Pandects pa. 91, gives several contrivances for these instruments. Solomon de Caus also treats on this subject in his Reasons of Moving Forces &c. So also does Ozanam, in his Mathematical Recreations, in which is contained a Treatise on Elementary Clocks, translated from the Italian of Dominique Martinelli. There is likewise a treatise on Hour-Glasses by Arcangelo Maria Radi, called Nova Sceinza de Horologi Polvere. See also the Technica Curiosa of Gasper Schottus; and Amonton's Remarques & Experiences Physiques sur la Construction d'une nouvelle Clepsydre, exempte des défauts des autres.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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