A timber arch and trunk sufficient for two and a half feet pipes, which would be sufficient to supply the city with water for twenty years, could be erected and covered for about 75,000 dollars, which is about 150,000 dollars less than estimated to complete the work of masonry. Such a bridge would last, if the cover was kept in good repair, probably 30 years. It is quite practicable to raise timber work to support the superstructure from near the water's edge but when it is considered that important bridges for travelled ;

roads, are usually supported by piers of stone, and aqueducts for canals almost invariably so, we could not, I think, do less than give that degree of permanence and durability which would be obtained by carrying up the piers as above suggested. In relation to the practicability and the diminished expense of erecting a timber arch, to support a trunk for the water pipes, there can be no question. At the same time there are objections to a timber structure, that should not be lost sight of. The decay that will be effected by the action of time on the timber, may not be sufficiently manifest in its external appearance, to give seasonable warning of its weakening influence on the structure, and (as is sometimes the case with wooden aqueducts on canals,) it will be liable to go down suddenly, and thus cause a breach in the aqueduct that will be difficult to repair in time to meet the wants of the city. The work may be destroyed by fire, as was the case with one of the Schuylkill bridges at Philadelphia. Its great elevation will expose it to severe storms of wind,