RW a P H e 107 the Aqueduct, would support iron pipes. The design was a bold one, yet instances where such bridges have been con- structed for road-ways afford examples of the feasibility and permanency of the structures, and prove that the application of that principle for this purpose was riot a visionary project. The plan which was adopted as the most suitable under all the considerations of economy and security to the work, was a Low Bridge to support an inverted syphon of iron pipes ; and the design of it was as follows : adjacent to the southern shore of the river there was to be constructed an arch for the channel of the river, of 80 feet span and spring- ing from abutments 10 feet above high water level ; this would form a passage of 80 feet wide, and the height from high water level to the under side of the arch at the crown would be 50 feet : south of this arch followed three other arches on the slope of the rocky hill, of 35, 30, and 25 feet span : south of these arches a foundation wall was designed to continue the plane of inclination to the level of the Aque- duct. From the large arch to the northern shore of the river an embankment of stone was designed for the support of the pipes, and from this wall the table land on the northern shore and the slope of the northern side of the valley, would be excavated to a form to give the proper position to the pipes descending from the Aqueduct. The lowest level of the top of this stone embankment was designed to be 4 feet above flood tide. Suitable parapet walls were designed to be built along the sides of the embankment to sustain a covering of earth over the pipes. With the form which was given to this inverted syphon, four pipes, each of 3 feet inte- rior diameter, were found to give a discharge of water equal 108 to that of the Aqueduct of masonry on the established incli- nation. In accordance with this plan of the Low Bridge the work for crossing the River was put under contract and some pro- gress made in its execution, when a law was passed by the Legislature of the State requiring, instead of this, a structure, the arches of which should be (over the channel of the river) at least 80 feet span and having a distance of 100 feet from the level of high water to the under side of the crown ; or to go under the channel of the river by a structure which should not rise above the bed, and that would leave the present channel unobstructed. 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 Le- gislature. 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 know- ledge 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 109 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.