Chap. IV.

ROMAN.

193

braced the whole of what hadbeen lost in the interior of thecrater, by the rise in the surface ofthe lake.

The nature of the stone cutthrough to form this tunnel madethe execution a task of great dif-ficulty. It is a lava as hard asiron, through which a passage wasbroken, high enough for a man towalk in it, 3 feet 6 inches broad,and 6000 feet long. On the lineof its course, fifty shafts weresunk to the bottom of the pro-jected tunnel, whereby its leveland direction were accurately keptand determined. After these shaftswere sunk, the workmen com-menced cutting away till they met,and the stone was lifted out bymeans applied at the top of theshafts. When the tunnel wasnearly finished, and a thin parti-tion only separated it from thelake, a small hole was madethrough it, -which let off the waterby degrees, and enabled the workmen

ELEVATION

Fig. 22)

AKCH OP THE EMISS.UUCM

to construct the wall of masonry around the mouth.

^.'^7)7;

w-Sw'

Fig. 222.

LAKE ALBANO.