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 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 water level. This would not carry the work up to the level of the Aqueduct, and would render it necessary to connect the Aqueduct on each side of the valley by iron pipes which would descend to the level of the bridge and crossing it rise again to the masonry channel-way. The plans which were before spoken of for a bridge of masonry across this valley, contemplated a structure which would maintain the regular inclination of the Aqueduct; and the channel-way would have been formed of masonry having a cast iron lining; but a more full consideration of the subject suggested the propriety of using iron pipes over the bridge, even if it had been carried up to the grade plane of the Aqueduct: when the use of iron pipes was determined upon, then considerations of economy induced them to build the work _only_ high enough to comply with the requirements of the law. The plan which has been adopted for building an Aqueduct bridge across this valley is as follows: on the south shore of the river there is one arch of 50 feet span, across the river there are eight arches, each of 80 feet span, and on the north shore there are six arches each of 50 feet span; making a range of fifteen arches. From the extremes of this range of arches, a foundation wall of dry stone work connects with the Aqueduct. Two of the piers in the river have a rock foundation and the foundations for those where rock is not obtained is formed by driving piles which are placed 2½ feet from centre to centre, and the spaces between filled with concrete to a depth of 3 feet below the top of them.