Chap. IV.
ROMAN.
Ml
In a picture found at Pompeii is the representation of one of these open moles withseven arches, which allow the sea to pass freely under them; the upper part or road is
a w a/
decorated with trophies. There is also shown an island with various buildings con-structed on it, probably for the purpose of containing the stores required in the equipmentof vessels, or for the residence of the guardians of the port.
At the end of the fifteen arches of the mole at Pozzuoli was the light-house, of which there are now no remains. In setting out the openingsor piers which sustained the arches, their span -was made equal to theirdepth, the piers being in width half as much more as that of the apertures;thus great strength was obtained, and the force of the waves beating againstsuch a structure would lose a part of their power, being allowed a passagethrough to the other side.
The section through one of the arches of this mole exhibits the portiontowards the sea, and by the side is shown the level at which the waterusually stood with reference to the arches, which were not semicircular.
The great distance that some of these open moles were continued into thesea, where there w’as a considerable depth of water, would lead us to imagine
Fig. 151. PLAN AND ELEVATION OP POZZUOLI.
that the piers could not have been constructed without the aid of very strong cofferdams ;and we learn that such were used resembling ships, and sunk where the work was to beexecuted. There can be no doubt that the Roman engineers were accustomed to drivepiles in deep water, as the descriptions left us of Caesar’s and Trajan ’s bridges fullyprove: to unite these piles as a barrier against the sea, and afterwards pump out thewater by the machines in use, would not be a difficult task: the tide in the Mediter-ranean does not rise sufficiently to interrupt the progress of such undertakings, andwhere the faces of the piers are worked in regular courses of masonry, no othermethod could have been adopted : by heightening the sides of one of their large galleys,a carpoon might be formed, in which they could pile up their material, and thus effecttheir object.