192

HISTORY OF ENGINEERING.

Boor I.

manner; consequently, fearingthe pestilential effects of sucha vast pool, the earth wasagain thrown in, and all thesubstructions of this interest-ing building hidden from sight.

This sewer is not of largerdimensions than another dis-covered in 1742, which passedunder the Comitium andForum, 40 palms below thepresent surface ; every part ofthe imperial city had its sewers,and many are formed of pep-perino stone, so universallyused before the introduction ofthe travertine, which did nottake place till long after. TheBocca della Verita is a largemarble covering to one of thesewers of the Forum, whichadmitted the surface waterinto it. When we recollectthat the quantity of waterconveyed by the aqueductsinto the imperial city sur-

Fig. 218.

passed that which any modern town receives,we naturally enquire by what means the bathsand fountains were cleansed, or the surplusquantity carried off; and on examining thevarious buildings, we find admirable and ampleprovision for their draining, and what is of farmore importance, a dimension given to thesewers sufficient to render them effective; theirsectional area was increased as they advanced,and there appears to have been in Rome onegeneral direction, which proportioned the partsas well as the whole. Had Rome been di-vided into districts, each with its board of di-rectors, sewers might have been built, as inLondon , like an inverted telescope, growingless as they advanced, instead of greater, ortwo sewers might have been conducted intoone, whose sectional area was not equal toeither.

SECTIONS OF ROMAN SEWERS.

Fig. 219- BOCCA bELLA VERITA.

Alban Lake -The plan for piercing

directing the course of the stream,which was running over on thelowest side of the lake into a re-gular channel, was adopted fortwo reasons ; first, as a preventiveagainst the violent floods thatwould have taken place wheneverthe waters received any extraor-dinary increase; and, secondly, asthe space between the level atwhich the lake overflowed andthat of the tunnel, where thebanks arc six miles round, was ofgreat value, even supposing thatthe land in those days, as now,was employed to grow wood. Theobject was not to gain new land,but to recover what the pro-prietors had been deprived of;indeed, that which was regainedmay perhaps not even have em-

a tunnel through the wall of the lava, instead of