[A very ingenious application of the principle of the clepsydra, for the purpose of measuring accurately very small intervals of time, is due to the late Captain H. Kater. Mercury is allowed to flow from a small orifice in the bottom of a vessel, kept constantly filled to a certain height. At the moment of noting any event, the stream is interrupted and turned aside into a receiver, into which it continues to run till the moment of noting any other event, when the intercepting cause is suddenly removed. The stream then flows in its original course. The weight of mercury in the receiver, compared with the weight of that which passes through the orifice in a given time, observed by the clock, gives the interval between the events.] FOOTNOTES [197] [Sextus Empiricus (Adv. Math. cap. 21) says that the Chaldæans divided the zodiac into 12 equal parts, as they supposed, by allowing water to run out of a small orifice during the whole revolution of a star, and dividing the fluid into 12 equal parts, the time answering to each part being taken for that of the passage of a sign over the horizon.] [198] Lib. ix. c. 9. [199] [Some mode of measuring time by the reflux of water, however rude it might be, was used at Athens before the time of Ctesibius, as we see by various passages in Demosthenes.] [200] Auctor Dialog. de Caus. Cor. Eloq. 38.--The orators were confined to a certain time; and hence Cicero says, _latrare ad clepsydram_. [201] Some account of the writers who have spoken of the water-clocks of the ancients may be found in Fabricii Bibliograph. Antiquaria, p. 1011. They were formerly used for astronomical observations. The authors who treat of them in this respect are mentioned in Riccioli Almagest. Novo, i. p. 117. [202] In that year Kircher’s Ars Umbræ et Lucis was published for the first time. In the edition of 1671, several kinds of water-clocks are described, p. 698. [203] A particular account of these water-clocks is to be found in Ozanam, Recréations Math. et Physiques [republished in Hutton’s Mathematical Recreations, ii. 40]. Bion on Mathematical Instruments. [204] Muschenbroek, Philos. Natur. i. p. 143. [205] Journal des Sçavans, 1691. [206] This monk may be considered as the restorer of the clepsydra, or clock which measures time by the fall of a certain quantity of water confined in a cylindric vessel. These clocks were in use among ancient nations. They are said to have been invented at the time when the Ptolemies reigned in Egypt. Dom Vailly, who applied himself particularly to practical mathematics, having remarked the faults of these clocks, bestowed much labour in order to bring them to perfection; and by a number of experiments, combinations, and calculations, he was at length able to carry them to that which they have attained at present. At the time of their arrival they were very much in vogue in France.--Hist. Littéraire de la Congr. de St. Maur, ordre de S. Bénoit. Bruxelles, 1770, 4to, p. 478.