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 exter- nal 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 my power, to produce a successful accomplishment. It is, however, my opinion, as before expressed, that under all the circumstances, the aqueduct 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 completed, it will be viewed as the most satisfactory work. Respectfully submitted. JOHN B. JERVIS, Chief Engineer, N. Y. W. W. 172 MEMOIR OF THE The Commissioners, for all sufficient reasons, preferred the high bridge to the tunnel, and without having in any degree changed their own views, as to the superiority of the syphon bridge to either, they issued a notice on 15th June for building a bridge of ma- sonry, in conformity with the specifications exhibited. Meantime the general work on the line went forward " with full as much speed," say the Commissioners, " as is desirable under present circumstances ;" they add, " had the Commissioners, however, been left to pursue the original plan of the work, there can scarcely be a doubt that the Croton water would have been flowing through our streets, and regaling both the taste and sight of our citizens, on, if not before, the 4th July, 1842." The summary of the work for the first six months of 1839, is thus given : 1st. THE AQUEDUCT. — The whole length of aqueduct, arched complete, is 77,587 feet, running measure, or about 14 miles and two-thirds. That completed on the first of January last, as per our semi-annual report of that date, was 59,169 feet, or 11 miles and one-fifth. The increase in this part of the work, since our last report, is 18,418 feet, or about three and a half miles in length. 2d. SIDE WALL. — The length of side wall ready to receive the brick facing and arches, was, at our last report, 1~443 feet. It now amounts to 17,535 feet, making an increase in this description of the work, of 16,092 feet, in six months. 3d. CULVERTS. — There are 92 culverts now complete and in use, leading off the water of the several streams at which they are the conduits. Their