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HOROLOGIUM (upc>\6ytov) was the name of the various instruments by means of which the ancients measured the time of the day and night. The earliest and simplest horologia of which mention is made, were called TrdAos and yvw^v. Herodotus (ii. 109) ascribes their invention to the Babylonians ; Phavorinus (ap. Diog. Laert. ii. 1. 3 ; compare Suidas, s. v. IVaj/xajj/ and 3Ava^i/j.av-Spos) to Anaximander ; and Pliny (H. N. ii. 76) to his disciple Anaximenes. Herodotus mentions the 7r6\os and yv&iiwv as two distinct instruments. Both, however, divided the day into twelve equal parts, and were a kind of sun-dial. The yv&iJLuiv, which was also called o-roi%e?oj', was the more simple of the two, and probably the more ancient. It consisted of a staff or pillar standing perpendicular, in a place exposed to the sun ((rKidOypov), so that the length of its shadow might be easily ascertained. The shadow of the gnomon was measured by feet, which were probably marked on the place where the shadow fell. (Hesych. s. «?. Eirrdirovs ffKia and 5it?5eKa7ro5os; Pollux, i. 72.) The gnomon is almost without exception mentioned in connection with the Stiirvov or the bath ; and the time for the former was towards sunset, or at the time when the shadow of the gnomon measured ten or twelve feet. (Aristoph. Eecles. 652, with the Schol. ; Pollux, 1. c. ; Menander, ap. Ailien. vi. p. 243 ; Hesych. s. v. AeKcforouj/ 2Tot%eroj>.) The longest shadow of the gnomon, at sunrise and sunset, was generally 12 feet, but in some cases 24 feet, so that at the time of the Setirvov it was 20 feet. (Eubulides, ap. Athen. i. p. 8.) The time for bathing was when the gnomon threw a shadow of six feet. (Lucian, Cronos. c. 17, Somn. s. Gall. e. 9.) In later times the name gnomon was applied to any kind of sun-dial, especiall3r its finger, which threw the shadow, and thus pointed to the hour. Even the clepsydra is sometimes called gnomon. (Athen. ii. p. 42.)
The gnomon was evidently a very imperfect instrument, and it was impossible to divide the day into twelve equal spaces by it. This may be the reason that we find it only used for such purposes as are mentioned above. The ir6\os or yXiorpo-thoi/, on the other hand, seems to have been a more perfect kind of sun-dial ; but it appears, nevertheless, not to have been much used, as it is but seldom mentioned. (Aristoph. ap. Polluc. ix. 5.) It consisted of a basin (Ae/mm), in the middle of which the perpendicular staff or finger (jv&ijlwv) was erected, and in it the twelve parts of the day were marked by lines. (Alciphron, Epist. iii. 4 ; Lucian, Leaciph. c. 4.)
Another kind of horologium was the clepsydra (KXetyvdpa). It derived its name from KXsirrsw and 85cup, as in its original and simple form i't consisted of a vessel with several little openings (rpviT^]j,ara) at the bottom, through which the water contained in it escaped, as it were, by stealth. This instrument seems 'as first to have "been used only for the purpose of measuring the time during which persons were allowed to speak in the courts of justice at Athens. The time of its invention or introduction is not known ; but in the age of Aristophanes (see Aclmrn. 653, Vesp. 93 and 827) it appears to have been in common use. Its form and construction may be seen very clearly from a passage of Aristotle (Problem, xvi. 8). The clepsydra was a hollow globe, probably somewhat flat at the top part, where it had a short
neck (cwAJs), like that of a bottle, through which the water was poured into it. This opening might be closed by a lid or stopper (tt&j/xo), to prevent the water running out at the bottom. The clepsydra which Aristotle had in view was probably not of glass or of any transparent material, but of bronze or brass, so that it could not be seen in the clepsydra itself what quantity of water had escaped. As the time for speaking in the Athenian courts was thus measured by water, the orators frequently use the term vSap instead of the time allowed to them (eV t$ e/^ u5aTt5 Demosth. de Coron. p. 274 ; eav fJX^Plf T^ #5&y>, c. Leocli. p. 1094). Aeschines (c. Ctesiph. p. 587), when describing the order in which the several parties were allowed to speak, says that the first water was given to the accuser, the second to the accused, and the third to the judges. An especial officer (6 e<£' #5o»p) was appointed in the courts for the purpose of watching the clepsydra, and stopping it when any documents were read, whereby the speaker was interrupted ; and it is to this officer that Demosthenes (c. StepJi. i. p. 1103) calls out : (tv 6e eViAaSe rb vficap. The time, and consequently the quantity of water allowed to a speaker depended upon the importance of the case ; and we are informed that in a ypatyfy irapairpea'&eias the water allowed to each party amounted to eleven amphorae (Aeschin. de Pals. Leg. § 126), whereas in trials concerning the right of inheritance only one amphora was allowed. (Demosth. c. Macart. p. 1502.) Those actions in which the time was thus measured to the speakers are called by Pollux (viii. 113) Siitat irpbs #8<»p : others are termed Si/cat &vev uSaros, and in these the speakers were not tied down to a certain space of time. The only instance of this kind of actions of which we know, is the ypaty}) KaKcbffews (Harpocrat. 5. v. /cci/ccocm).
The clepsydra used in the courts of justice however was, properly speaking, no horologium ; but smaller ones, made of glass, and of the same simple structure, were undoubtedly used very early in families for the purposes of ordinary life, and for dividing the day into twelve equal parts. In these glass-clepsydrae the division into twelve parts must have been visible, either on the glass-globe itself, or in the basin into which the water flowed. . These instruments, however, did not show the time quite correctly all the year round ; first, because the water ran out of the clepsydra sometimes quicker and sometimes slower, according to the different temperature of the water (Athen. ii. p. 42 ; Pint. Quaest. Natur. c. 7) ; and secondly, because the length of the hours varied in the different seasons of the year. To remove the second of these defects the inside of the clepsydra was covered with a coat of wax during the shorter days, and when they became longer the wax was gradually taken away again. (Aen. Tact. c. 22.) Plato is said to have used a WKTcpivkv Sopo\6yiov in the shape of a large clepsydra, which indicated the hours of the night, and seems to have been of a complicated structure. (Athen. iv. p. 174.) This 'instance shows that at an early period improvements were made on the old and simple clepsydra. But all these improvements were excelled by the ingenious invention of Ctesibius, a celebrated mathematician of Alexandria (about 135 b. c.). It is called j &>poXoywv vfipavXiKbv, and is described by Vitru-J vius (ix. 9 ; compare Athen. 1. r.). Water was
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