206 MEMOIR OF THE walls is also increased, and the proportion of cement to sand in concrete and mortar for stone work is one to two and a half. In other respects the masonry in conduit, is similar on foundation walls to that in excavation. The proportion of lime of aqueduct masonry on foundation walls over valleys, to that in excavation, is about as one to eight. The masonry of the aqueduct is covered with earth to a sufficient depth to protect it from frost. Culverts, To pass the streams that intersect the line, and the land floods, there has been constructed under the aqueduct 114 culverts, whose aggregate length is 7959 feet. The span varies from one and a half foot to twenty-five feet. Those of one and a half foot span are of a square form, and are constructed by laying down a foundation of concrete masonry, on which a course of well jointed stone, not less than nine inches thick is laid, forming a stone platform on which well dressed stone is laid, one or two courses high, for the abntments or side walls, and finished by a second course of well dressed flagging, not less than nine inches thick all the work laid in hydraulic mortar. ; The culverts over one and a half foot span, with the exception of three of twenty-five feet span, are constructed as follows :

A foundation of concrete masonry is formed, and in a few instances some timber and plank is used with it on this an inverted arch of well dressed stone is laid in regular ;