498 ANCIENT HYDRAULIC WORKS. 
about six miles in circuit, occupies one of the craters of an ex- 
tinct volcanic range, and the surface of its waters is about nine 
hundred feet above the sea. It is fed by rivulets and subterra- 
nean springs originating in the Alban Mount, or Monte Cavo, 
the most elevated 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 reg- 
ularly 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 subterra- 
nean channels, considerably below 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 supplies 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 was pierced at, 
a level much below the height to which it had risen. This gal- 
lery, cut entirely with the chisel through the rock for a distance 
of six thousand feet, or nearly a mile and one-seventh, is still in 
so good condition as to serve its original purpose. The fact 
that this work was contemporaneous with the siege of Veil, 
has given to ancient annalists occasion to connect the two 
events, but modern critics are inclined to reject Livy’s account 
of the matter, as one of the many improbable fables which dis- 
figure the pages of that historian. It is, however, repeated by 
Cicero and by Dionysius of Halicarnassus, and it is by no means 
impossible that, in an age when priests and soothsayers monopo- 
lized both the arts of natural magic and the little which yet 
existed of physical science, the Government of Rome, by their 
aid, availed itself at once of the superstition and of the mili- 
tary ardor of its citizens to obtain their sanction to an enter- 
prise which sounder arguments might not have induced them 
to approve. 
Still more remarkable is the tunnel cut by the Emperor 
