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 p457  Emissarium

Article by Anthony Rich, Jun. B.A. of Caius College, Cambridge
on pp457‑458 of

William Smith, D.C.L., LL.D.:
A Dictionary of Greek and Roman Antiquities, John Murray, London, 1875.

EMISSA′RIUM (ὑπόνομος), a channel, natural or artificial, by which an outlet is formed to carry off any stagnant body of water (Plin. H. N. XXXIII.4 s.21; Cic. ad Fam. XVI.18). Such channels may be either open or underground; but the most remarkable works of the kind are of the latter description, as they carry off the waters of lakes surrounded by hills. In Greece, the most remarkable example is presented by the subterraneous channels which carry off the waters of the lake Copais into the Cephisus, which were partly natural and partly artificial (Strab. IX. p406; Thiersch, État actuel de la Grèce, vol. II p23; Müller, Orchomenos, pp49, &c., 2nd ed.).

Another specimen of such works among the Greeks at an early period is presented by the subterraneous channels constructed by Phaeax at Agrigentum in Sicily, to drain the city, about B.C. 480; which were admired for their magnitude, although the workmanship was very rude (Diod. Sic. XI.25).

Some works of this kind are among the most remarkable efforts of Roman ingenuity. Remains still exist to show that the lakes Trasimene, Albano, Nemi, and Fucino, were all drained by means of emissaria, the last of which is still nearly perfect, and open to inspection, having been partially cleared by the present king of Naples. Julius Caesar is said to have first conceived the idea of this stupendous undertaking (Suet. Jul. 44), which was carried into effect by the Emperor Claudius (Tacit. Ann. XII.57).a

An engraving of a manmade waterfall. It depicts an ancient Roman water discharge into the Garigliano river.
The following account of the works, from observations on the spot, will give some idea of their extent and difficulties. The circumference of the lake, including the bays and promontories, is about thirty miles in extent. The length of the emissary, which lies nearly in a direct line from the lake to the river Liris (Garigliano), is something more than three miles. The number of workmen employed was 30,000, and the time occupied in the work eleven years (Suet. Claud. 20; cf. Plin. H. N. XXXVI.15 s.24 §11). For more than a mile the tunnel is carried under a mountain, of which the highest part is 1000 feet above the level of the lake, and through a stratum of rocky formation (carnelian) so hard that every inch required to be worked by the chisel. The remaining portion runs through a softer soil, not much below the level of the earth, and is vaulted with brick. Perpendicular openings (putei) are sunk at various distances into the tunnel, through which all the excavations were partly discharged and a number of lateral shafts (cuniculi), some of which separate themselves into two branches, one above the other, are likewise directed into it, the lowest at an elevation of five feet from the bottom. Through these the materials excavated were also carried out. Their object was to enable the prodigious multitude of 30,000 men to carry on their operations at the same time, without incommoding one another. The immediate mouth of the tunnel is some distance from the present margin of the lake, which space is occupied by two ample reservoirs, intended to break the rush of water before it entered the emissary, connected by a narrow passage, in which were placed the sluices (epistomia). The mouth of the tunnel itself consists of a splendid archway of the Doric order, nineteen feet high and nine wide, formed out of large blocks of stone, resembling in construction the works of the Claudian aqueduct. That through which the waters discharged themselves into the Liris was more simple, and is represented in the preceding woodcut. The river lies in a ravine between the arch and foreground, at a depth of 60 feet below, and consequently cannot be seen in the cut. The small aperture above the embouchure is one of the cuniculi above mentioned. It appears that the actual drainage was relinquished soon after the death of Claudius, either from the perversity of Nero, as the  p458 words of Pliny (l.c.) seem to imply, or by neglect; for it was reopened by Hadrian (Spart. Hadr. 22). For further information see Hirt, who gives a series of plans and sections of the works connected with the Lacus Fucinus (Gebäude d. Griech. u. Röm. pp371‑375, Pl. XXXI figs. 14‑21).

Thayer's Note:

a This is most misleading. Although the Fucine lake was drained, it wasn't until 1878. In one of the rare failures of Roman engineering, after several years of work and major expenditure, the drainage scheme, inaugurated by Claudius himself, proved quite embarrassing: the water level dropped by about 4 meters and stabilized, leaving the lake very much there.

An excellent detailed 40‑page article on both drainage works can be found at Aercalor. In Italian, with a wealth of photographs, maps, diagrams, historical and topographical data, and a bibliography.

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