In the commencement of the work, there was much difficulty in getting the workmen to lay their stone and brick in a thorough, full bed of mortar, which is obviously very important in hydraulic masonry. But a rigid system of inspection, requiring all imperfect work to be taken down and relaid, was successful in obtaining work of great compactness and solidity. The area of a cross-section of masonry in the aqueduct is :

Concrete masonry, - - 4.605 ) Stone in side walls, 21.572 V Do. inspandrils, - 2.690^28.867, Brick in arches and side facing, - - 13.658,

- Total, 42,525 sqr. feet

A limited departure from the above area has been made where peculiar circumstances seemed to justify or require it the most important in extent is, where the aqueduct passes ;

over low grounds or valleys, and a dry wall of stone is raised to the proper level to support the conduit masonry, and generally for the depressed bottom on the 4.949 miles at the upper end of the aqueduct, and in a few other cases in the first contracts, where the bottom arch and brick facing is 8 inches thick. On the foundation walls, the concrete masonry is laid one foot extra thickness, and three feet extra width the base of the side ;

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.