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,

which, as it grows weak from age, may at a time not expected, force it from the piers, or so derange its joints as to render it incapable of supporting the aqueduct. All other work constructing on the line has been designed with a view to the greatest durability, and to guard as much as possible against the necessity for repairs, and especially such as would materially jeopardise the regular flow of water in the aqueduct. To adopt a wooden structure at the most important point, in regard to its magnitude and expense, would certainly be a great departure from all other work on the line of aqueduct and to the ;

undersigned it does not appear expedient, for the difference that may be saved in the first instance, to change the character of the work by adopting a wooden structure at this place. Should the Board be of opinion, that either the tunnel plan, or the wooden arch bridge, would be the most suitable for them to adopt, I shall feel no hesitation in going forward, with every effort in mypower, to produce a successful accomplishment. It is, however, opinion, as before expressed, that under all the circumstances, the aqueduct my bridge of masonry with iron pipes for the conduit, is the most suitable structure to be adopted and should the Board coincide with this opinion, I have no doubt, that when ;