116

HISTORY OF ENGINEERING.

Book I.

arriving at the first step above the pavement of the gallery of the half-story, was received intothe perpendicular hole with which this first step is pierced, then fell into the cistern placedbelow, ran down the gargouille, at the extremity of which it fell into the chambers underthe great staircase, where it was quickly absorbed. If the abundance of water was suchthat the first step could not receive it, the excess fell on the stone placed below on a levelwith the pavement of the gallery, where it followed the same course as the rest, and ranunder the same vault by a similar contrivance, so that it was impossible any could comeinto the half-story gallery.

The water which the wind drove into the first-floor gallery, through the openings of theporticoes, fell on the first step, by which it descended to the half-story gallery. There itwas rectived into the hollows of the first step, at the extremity of which were two holes aninch in diameter, below which were stone gutters communicating with discharges made oneach side of the recesses of this staircase below the winders just mentioned. 'Hie waterthen followed the course indicated above : a double advantage was thus obtained, that ofcleansing the urinals and getting rid of the water, which, without this precaution, would haveinundated the inner gallery.

'The great discharges of the second flight of steps, fifty-six in number, received the waterof the vomitories of the third and fourth precinct.

The construction of the great outer aqueduct, which brought the water from the fountainof Nismes, is next to be considered.

It was discovered when excavating in the middle of the outer gallery of the groundfloor, opposite the north gate, where an aqueduct was supposed to exist under thisgallery and throughout its circular development. This great aqueduct has a breadthof 32 inches, and a height of 6 feet from the bottom to the under side of the key-stonewhich covers it. lake all the others, it was filled with earth and mud up to the top :on the north side it was found perfectly preserved, built of tooled stone, as was thesemicircular arch which covers it. This vault has been replaced by landings of freestone,where it crosses the north passage from the door of the present guard-house to the circularaqueduct of the arena. It traverses the arena to the centre of the ellipsis, then deviates tothe south-west, leaving the building at the sixth portico east of the south door. Its con-struction, precisely the same as that on the north side, is covered with landings. Theexcavation was continued from where it issues from the amphitheatre, for a length of 40 feet,at which distance it was broken and could not be further traced.

Two manholes for descending into this aqueduct existed in the outer gallery on theground floor, one at its entrance at the north gate, the other at its issue through the sixthportico west of the south gate. These manholes are perfectly preserved : they are 2 feetsquare, covered with a landing: below the pavement a square hole 30 inches wide,by 16 inches high, is cut, 3 feet 6 inches above the bottom, on each side of the aqueduct, inthe middle of the outer gallery on the ground floor, to allow the water which filteredthrough the pavement to run into the great aqueduct.

Three quarries were worked by the Romans to obtain the stone requisite for the ex-ternal works of this amphitheatre : the greater part of the seats, and facing of the podium ofthe first and second precinct, and some portion of the interior arcades, were of stone, broughtfrom the quarries of Baruthel ; some steps and the worked moellon came from Roque-maillere. Nearly the whole of the arcades of the lower story, the entresol, the first andsecond story, are executed in the stone from the quarries of the Pont du Gard . They areall calcareous, and of a grain more or less fine. That from the quarries of Baruthel is compact,firm ani fine; its weight about 515 lbs. per cubic foot: when employed as it is foundin the bed, it is an excellent stone, in many instances where this precaution has beenomitted the courses are entirely decomposed.

The stone from lloquemaillere is stronger and harder, and weighs about 460 lbs. percubic foot; it is of an excellent quality, and has undergone little change.

That brought from the quarries of the Pont du Gard is a coarse grit, and stands wellwhen used internally : being porous, when subject to exposure or rain, it is greatly in-jured : a cubic foot weighs about 333 lbs. The difficulties arising from the obliquedirection of the whole of the constructions, externally and internally, are admirably over-come. The vaults of all the stories are executed with great precision, as are those of thecorridors; but the same care has not been taken in the construction of the arches of theexterior, which are oblique on their plan and elevation. The Romans in this example ofoblique work had not quite arrived at perfection, as they have not been sufficiently attentiveto the true setting out of each stone. All the freestone is laid in cement, and the bed cutwith the utmost precision ; a hole in the centre of gravity of each stone indicates that itwas lifted by the aid of the Lewis; and it appears, by the nicety of the workmanship, thatthe upper stone, previous to being bedded, was suspended by the Lewis, and, water beingthrown over the under bed, was then gradually lowered: all the rough particles thatmight remain between were brought down by friction ; and when the stones easily workedagainst each other, the upper was definitively fixed. By no other means can we account