The work is constructed of well dressed stone masonry, laid in hydraulic cement. Open hance walls are carried up over the interior and above the solid spandrels, and united at the top by brick arches. The spaces between the hance walls are carried entirely across the crown of the arch, to afford as much facility as possible for any water that might leak from the conduit to pass off. The depth of arch stone at the spring line is four feet, and at the crown three feet. The parapets and walls of the conduit are carried up with an opening of six inches between them, more effectually to guard against the effects of frost, and to carry off any water that might leak from the conduit, and prevent its being absorbed by the outer or parapet wall. The conduit over this bridge is constructed of stone and brick masonry, so arranged as to receive a lining of cast iron the iron being set back so as to receive a facing of brick ;

over the bottom and sides, which is further secured by three courses of hydraulic cement plastering. It is not probable that any considerable quantity of water would have escaped, if there had been no other security than the stone and brick masonry but it is hardly possible ;

to make masonry so perfectly impervious, that it will not show wet, or a sweating appearance on the exterior wall, and the action of frost in this climate, will produce in such cases a deterioration, that may be slow, but will eventually destroy the work hence the ;