As the two mountains forming the valley of Gardon are not of equal height at the points in the line of aqueduct, that on the left side of the river being lower than the level of the aqueduct, while the right side is more elevated, the conduit on one side is carried onwards by continuing the third range of arches, and on the other side, the range terminates in the side of the mountain.

The Pont du Garde is constructed entirely with hewn stone ; no rubble work is introduced even into the filling up of the piers, or spandrils of the arches. The masonry

PRELIMINARY ESSAY. 31

has been finished and put tog-ether without lime, or any other kind of cement, and owes itsstability to the mass of each block, and the precision of the faces in their beds and

joints. The canal of the aqueduct is in fact the only part which is not constructed with hewn stones r being made with a sort of jointed rubble on the outer and inner faces of the canal, but of the common rubble in the filling up. This work, where the cement has not been sparingly used, forms a mass absolutely impenetrable to any passage of the water. The inside facing of the walls, and the bottom hollowed in the form of an arc of a circle, were covered with a coat of cement about two inches in thickness, composed of quicklime, fine sand, and pulverised bricks. This cement is at the present day of a consistence equal to that of the hardest and most compact stone, and without the slightest crevice or flaw to be any where seen in it. This first coat of cement was covered with a second