(ὡρολόγιον). A name given to 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 πόλος and γνώμων. Herodotus (ii. 109), who ascribes their invention to the Babylonians, mentions the πόλος and γνώμων as two distinct instruments. Both, however, divided the day into twelve parts, and were a kind of sun dial. The γνωμων, which was also called στοιχεῖον, 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 (σκιάθηρον), 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 ( Poll.i. 72). The gnomon is almost without exception mentioned in connection with the δεῖπνον 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 ( Eccles. 652, with the Schol.; Poll. l. c.). The longest shadow of the gnomon, at sunrise and sunset, was twelve feet. The time for bathing was when the gnomon threw a shadow of six feet (Lucian, Cronos, c. 17; Somn. s. Gall. c. 9). In later times the name gnomon was applied to any kind of sundial, and especially to 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. The πόλος or ἡλιοτρόπιον, on the other hand, seems to have been a more perfect kind of sundial; but it appears, nevertheless, not to have been much used, as it is but seldom mentioned ( Aristoph. ap. Poll.ix. 46). It consisted of a basin (λεκανίς), in the middle of which the perpendicular staff or finger (γνώμων) was erected, and in it the twelve parts of the day were marked by lines (Lucian, Lexiph. 4).

Another kind of horologium was the clepsydra (κλεψύδρα). It derived its name from κλέπτειν and ὕδωρ, as in its original and simple form it consisted of a vessel with several little openings (τρυπήματα) at the bottom, through which the water contained in it escaped, as it were, by stealth. This instrument seems at 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 Acharn. 692) 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 (αὐλός), like that of a bottle, through which the water was poured into it. This opening might be closed by a lid or stopper (πῶμα), 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 ὕδωρ instead of the time allowed to them (ἐν τῷ ἐμῷ ὕδατι, Demosth. De Coron. p. 274. 139). Aeschines (c. Ctesiph. 197), 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 (ὁ ἐφ ὕδωρ) 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. 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 γραφὴ παραπρεσβείας the water allowed to each party amounted to eleven amphorae ( De Fals. Leg. 126), whereas in trials concerning the right of inheritance only one amphora was allowed (Demosth. c. Macart. p. 1052. 8). Those actions in which the time was thus measured to the speakers are called by Pollux (viii. 113) δίκαι πρὸς ὕδωρ: others are termed δίκαι ἄνευ ὕδατος, and in these the speakers were not tied down to a certain space of time.

The clepsydra used in the courts of justice, however, was, properly speaking, not a 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 ( Quaest. Nat. 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. 10). Plato is said to have used a νυκτερινὸν ὡρολόγιον in the shape of a large clepsydra, which indicated the hours of the night, and seems to have been of a complicated structure. 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 (q.v.), a celebrated mathematician of Alexandria (about B.C. 135). It is called ὡρολόγιον ὑδραυλικόν, and is described by Vitruvius (ix. 9). Water was made to drop upon wheels which were thereby turned. The regular movement of these wheels was communicated to a small statue, which, gradually rising, pointed with a little stick to the hours marked on a pillar which was attached to the mechanism. It indicated the hours regularly throughout the year, but still required to be often attended to and regulated. This complicated clepsydra seems never to have come into general use, and was probably found only in the houses of very wealthy persons. The sundial or gnomon, and a simpler kind of clepsydra, on the other hand, were much used down to a very late period. The twelve parts of the day were not designated by the name ὥρα until the time of the Alexandrian astronomers, and even then the old and vague divisions, described in the article Dies, were preferred in the affairs of common life. At the time of the geographer Hipparchus, however (about B.C. 150), it seems to have been very common to reckon by hours.

There is still existing, though in ruins, a horological building, which is one of the most interesting monuments at Athens. It is the structure formerly called the Tower of the Winds, but now known as the Horological Monument of Andronicus Cyrrhestes. It is expressly called horologium by Varro (R. R. iii. 5. 17). This building is fully described by Vitruvius (i. 6. 4), and the following illustration shows its ground-plan.

Tower of the Winds

For the elevation see the article Andronicus. ]

The structure is octagonal, with its faces to the points of the compass. On the northeast and northwest sides are distyle Corinthian porticos, giving access to the interior; and to the south wall is affixed a sort of turret, forming three-quarters of a circle, to contain the cistern which supplied water to the clepsydra in the interior. On the summit of the building was a bronze figure of a Triton, holding a wand in his hand; and this figure turned on a pivot, so that the wand always pointed above that side of the building which faced the wind then blowing. The directions of the several faces were indicated by figures of the eight winds on the frieze of the entablature. On the plain wall below the entablature of each face, lines are still visible which, with the gnomons that stood out above them, formed a series of sundials. In the centre of the interior of the building was a clepsydra, the remains of which are still visible, and are shown on the plan, where the dark lines represent the channels for the water, which was supplied from the turret on the south, and escaped by the hole in the centre. Three other Athenian horologia are extant, one in the monument of Thrasyllus, another that of Phaedrus in the British Museum (C. I. G. 522), a third in the theatre of Dionysus, besides others from different parts of Greece.

The first horologium with which the Romans became acquainted was the sundial (solarium, or horologium sciothericum), and was, according to some writers, brought to Rome by Papirius Cursor twelve years before the war with Pyrrhus, and placed before the temple of Quirinus (Pliny , Pliny H. N.vii. 213). Varro stated that it was brought to Rome from Catina in Sicily, at the time of the First Punic War, by the consul M. Valerius Messala, and erected on a column behind the Rostra. But this solarium, being made for a different latitude, did not show the time at Rome correctly. Ninety-nine years afterwards, the censor Q. Marcius Philippus erected by the side of the old solarium a new one, which was more carefully regulated according to the latitude of Rome. But as sundials, however perfect they might be, were useless when the sky was cloudy, P. Scipio Nasica, in his censorship, B.C. 159, established a public clepsydra, which indicated the hours both of day and night. This clepsydra was in after-times generally called solarium ( De N. D. ii. 34, 87). The word hora for hour was introduced at Rome at the time when the Romans became acquainted with the Greek horologia, and was in this signification well known at the time of Plautus (Pseud. 1307). After the time of Scipio Nasica, several horologia, chiefly solaria, seem to have been erected in various public places at Rome. A magnificent horologium was erected by Augustus in the Campus Martius. It was a gnomon in the shape of an obelisk; but Pliny ( Pliny H. N.xxxvi. 73) complains that in the course of time it had become incorrect. Horologia of various descriptions seem also to have been commonly kept by private individuals ( Ad Fam. xvi. 18, 3); and at the time of the emperors, the wealthy Romans used to keep slaves whose special duty it was to announce the hours of the day to their masters ( Juv.x. 216, with Mayor's note; Mart.viii. 67; Petron.26).

From the number of solaria which have been discovered in modern times in Italy (thirteen having been discovered in the neighbourhood of Rome alone), we must infer that they were very generally used among the ancients. The following illustrations represent one of the simplest horologia which have been discovered; it seems to bear great similarity to that the invention of which Vitruvius ascribes to Berosus. It was discovered in 1741, on the hill of Tusculum, and is described by Zuzzeri (Venice, 1746), and by G. H. Martini, in his Abhandlung von den Sonnenuhren der Alten, p. 49 (Leipzig, 1777).