At this time when the work was going on vigorously, they were compelled to abandon the plan which had been adopted, and devise one which would comply with the requirements of the law of the Legislature. A comparison was instituted between the plan of a tunnel under the bed of the river and that of a bridge of masonry at the required height above the river. The tunnel would be at least 300 feet long and the top of the masonry forming it, would be 18 feet below high water level. In this tunnel the iron pipes would pass under the River and would be protected from the salt water. An estimate of the cost of crossing by means of each plan was made, and the result was in favor of the tunnel under the bed of the River ; but from the imperfect knowledge which could at best be obtained of the formation of the bed, there was great uncertainty in the estimate of the

cost of the tunnel and the time that would be required for its completion. The history of the progress of work in the tunnel under the Thames at London warned them of the

Ml

;

difficulties of such a work and the uncertainty of arriving at a proper estimate of the cost.

In the alternative to which they were driven by the Act of the Legislature, the plan of an Aqueduct bridge of masonry was adopted as the proper one for crossing the

River ; but in establishing its altitude they complied only with the requisitions of the law, and made the soffit or under side of the arches at the crown, 100 feet above common high