are vertical, and the top covered with a flagging of cut stone, except where it is under ground, in which situation the top is covered by an arch of stone. The interior face of the walls and the bottom were covered with a coat of plastering two inches in thickness, composed of quick-lime, fine sand, and brick nearly pulverized. This coating has now a tenacity and consistence equal to the hardest stone.

The size of the channel-way is the following : 4 feet wide and 5i feet high, except where the top is covered with an

arch, in which case it is H feet high in the interior.

The descent of the Aqueduct is 1 foot in 2500 feet, or 2^ feet per mile.

The water which flowed in this Aqueduct formed a deposit upon the sides, of lime, until nearly half the channel was closed; this deposit amounting to a thickness of 11 inches on each side. By the height of this deposit it has been ascertained that the water flowed generally with a depth of 3f feet.

The pont du Gard is that part of the Aqueduct of Nismes which crosses the deep valley in which runs the Gardon or Gard. This part, considered alone, is one of the noblest monuments built by the Romans among the Gauls. It is composed of three ranges of arches one above another. The first range, under which the Gardon flows, is formed by 6 arches ; the second by 11, and the third by 35, all of which are semicircular ; supported upon piers of greater or less height.