414 ANCIENT HYDEAULIC WOEKS. 



lake, now reduced by sedimentary deposit and tlie growth of 

 aquatic and semi-aquatic vegetation to the condition of a marsh, 

 was originally partially drained by natural subterranean outlets 

 in the underlying Kmestone rock, many of which still exist. But 

 these emissaries, or Tcata/ooihra^ as they are called in both ancient 

 and modern Greek, were insufficient for the discharge of the 

 water, and besides, they were constantly hable to be choked by 

 earth and vegetables, and in such cases the lake rose to a height 

 which produced much injury. To remedy this evil and secure a 

 great accession of fertile soil, at some period anterior to the ex- 

 istence of a written literature in Greece and ages before the time 

 of any prose author whose works have come down to us, two 

 tunnels, one of them four miles long, and of course not inferior 

 to the Torlonian emissary in length, were cut through the soKd 

 rock, and may stiU be followed throughout their whole extent. 

 They were repaired in the time of Alexander the Great, in the 

 fourth century before Christ, and their date was at that time tra- 

 ditionally referred to the reign of rulers who lived as early as the 

 period of the Trojan war. 



One of the best known hydraulic works of the Romans is the 

 tunnel which serves to discharge the surplus waters of the Lake 

 of Albano, about fourteen miles from Rome. This lake, about six 

 miles in circuit, occupies one of the craters of an extinct volcanic 

 range, and the surface of its waters is about nine hundred feet 

 above the sea. It is fed by rivulets and subterranean springs 

 originating in the Alban Mount, or Monte Cavo, the most ele- 

 vated peak of the volcanic group just mentioned, which rises to 

 the height of about three thousand feet. At present the lake has 

 no discoverable natural outlet, and it is not known that the water 

 ever stood at such a height as to flow regularly over the lip of 

 the crater. It seems that, at the earliest period of which we have 

 any authentic memorials, its level was usually kept, by evaporation 

 or by discharge through subterranean channels, considerably be- 

 low the rim of the basin which encompassed it ; but in the year 

 397 B.C., the water, either from the obstruction of such channels 

 or in consequence of increased suppKes from unknown sources, 

 rose to such a height as to flow over the edge of the crater, and 

 threaten inundation to the country below by bursting through its 

 walls. To obviate this danger, a tunnel for carrying off the water 



