Although the chief defects of the clepsydra were minimized by the use of the two vessels and by making the aperture through which the water ran of gold or some other substance which would wear away very slowly, yet there remained certain minor imperfections. The water could not be kept entirely from evaporating; it had to be emptied out at intervals and the reservoir refilled; its accuracy was affected by the expansion of the parts under change of temperature, or it might even freeze. These faults were obviated in the sand-glass or hour-glass which for short intervals of time was also more convenient. The clepsydra remained in use until clocks became superior to it in accuracy. See _Clocks, Interesting Old_; _Charlemagne_; _Vailly_. CLERKENWELL—A district on the north side of the city of London within the metropolitan borough of Finsbury. It is distinguished as one of the great centers of the watchmaking and jewelers' industries in England and long established there. The Northampton Polytechnic Institute, Northampton Square, has a department devoted to instruction in all branches of the trade. CLICK—The click, pawl, or dog, is a necessary accessory of a ratchet wheel. It is a finger, one end of which fits into the teeth of the ratchet, while the other is pivoted on its tangent. The ratchet is thus prevented from turning backward. CLOCK—Specifically, a time-piece not made to be carried about but to stand upon a shelf or table, hang upon a wall or as built into a tower. Formerly the term signified particularly a time-piece which struck the hours. The word has its origin in the word for _bell_ in Latin, _gloccio_; Teutonic, _glocke_; French, _cloche_; and Saxon, _clugga_. At one time the term was used to denote timekeepers driven by weights as distinguished from those driven by springs. CLOCK-WATCH—A watch which strikes the hours in succession, as distinguished from repeaters. Popular in the eighteenth century. [Illustration: CLOCK-BANJO] CLOCK, BANJO--A wall clock, so called from its shape, designed by Simon Willard, of Massachusetts and very popular in its time. [Illustration: CLOCK, BIRD-CAGE] CLOCK, BIRD-CAGE—An old form of English clock whose manufacture has been discontinued—it is the oldest form of English clock still doing service. Its main feature is the endless chain drive. These clocks run thirty hours. CLOCK, BRACKET—A form of clock very popular in England during the reign of Charles II, made to stand on a bracket or table and intended to be seen from all sides. These clocks had either a handle on top or one on each side. They were very beautifully finished. CLOCK, CANDLE—Wax or tallow candle, usually twelve inches long and marked with circular lines one inch apart. The candle would burn one inch every twenty minutes or three inches an hour. Invention credited to King Alfred the Great.