Engineering:Emissary (hydraulics)

An emissary (Latin emissarium, from ex and mittere, to send out) is a channel, natural or artificial, by which an outlet is formed to carry off any stagnant body of water. 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 Ancient Greece , the most remarkable examples are the subterranean channels that carry off the waters of Lake Copais into the Cephisus, which were partly natural and partly artificial; and those built about 480 BC by Phaeax at Agrigentum in Sicily to drain the city: they were admired for their sheer size, although the workmanship was crude.

The ancient Romans excelled in the construction of emissaries, as in all their hydraulic works, and remains are extant showing that lakes Trasimeno, Albano and Nemi were all drained by means of emissaries. The case of Lake Fucino is remarkable in two ways: the attempt to drain it was one of the rare failures of Roman engineering, and the emissary is now completely above ground and open to inspection. Julius Caesar is said to have first conceived the idea of this stupendous undertaking (Suet. Jul. 44); with the tunnels that bring his name, Claudius inaugurated what was to have been a complete drainage scheme (Tac. Ann. xii.57), but the water level dropped by just 4 meters and stabilized, leaving the lake very much there. Hadrian tried it again, but failed; and it was not until 1878 that Lake Fucino was finally drained.

The need for emissaries did not cease with Antiquity, of course, and modern examples abound.

The initial text of this article was an abridgment from Smith's Dictionary of Greek and Roman Antiquities (1875 edition, public domain).

External links

• Emissarium, the full article in Smith's Dictionary