be at least 15 per cent, greater than the calculated flow. I have not been much disappointed in finding the flow of water in the aqueduct, to exceed the calculation, as all my observations on the currents in canal feeders, have led me to believe the formulas laid down give rather less than the actual result. The flow of water through the pipes across Manhattan valley, and also the temporary pipe across Harlem river, being attended with circumstances somewhat different, has led some very intelligent persons to predict, that our expectations would not be realised in these cases ; it therefore maybe proper to observe that trial has proved such predictions to have been not well founded, as the flow through the pipes has in a very exact manner corroborated the anticipated capacity, as compared with that in the aqueduct. - The Croton reservoir, (which has received the name of Croton lake,) covers about 400 acres of land, find is available as a reservoir for 500,000,000 imperial gallons of water, above the level that would allow the aqueduct to discharge 35.000,000 gallons per day. The flow of the Croton is about 27,000,000 gallons in twenty -four hours at the lowest stages, which continues, with moderate rises by occasional rains, from two to three mouths in the year. This may be considered the minimum capacity of the river. When the wants of the city shall require a daily supply of 35,000,000 gallons, it will be neces- sary, during the season of lowest water, to draw daily from this reservoir 8,000,000 gal- lons, to make up the deficiency in the natural flow of the river. This amount the reser- voir would supply for 62 days, without any aid from occasional rains ; which may safely be relied upon, to keep up the required supply from the reservoir, beyond any drought we have ground to apprehend. The supply of the Croton, from its daily flow, aided by this reservoir, may therefore be taken witfrgreat confidence at 35,000,000 gallons, which will be very ample for the wants of the city for a long time to come ; and when the day arrives that it will require a larger quantity, it may be obtained by constructing other res- ervoirs further up the stream, where there are abundant facilities for such purposes." The total cost of the aqueduct, from the Croton dam to the distributing reservoir inclusive, will be nearly 9,000,000 of dollars, New- York, 27th July, 1842. The unfinished portions of the work, chiefly on the Harlem bridge, have been steadily advancing during the period since the introduction of the Croton into the city ; and by a letter of recent date from the chief engineer, we are informed that " the foundations for the two last water piers at Harlem bridge are now in progress ; the piles are driven for one of them, and they are nearly ready to receive the cap timbers and flooring on which the ma- sonry is to be constructed. For the other the piling is in progress. On the last founda- tion put down, the masonry is proceeding, and is nearly up to high water line. All the other piers are raised above high water, and the- masonry is actively progressing on se- veral of them. It is probable the unfinished foundations will be completed, and the ma- sonry raised above high water by the month of September, and several of the piers be carried up by the close of the season to their full height, ready to receive the arches." While these sheets are passing through the press, an examination of the whole line of aqueduct has been made with most satisfactory, results ; the water was stopped off be- * MEMOIR OF THE tween the Croton and the receiving reservoirs, and a minute personal inspection was made by the chief engineer and his assistants, who passed through the whole conduit, and only, upon very close examination, were enabled to detect some slight defects, which a few days would suffice for repairing. The exterior of the work had suffered as little injury as could be expected from the frosts of winter, and the heavy rains of this spring — and nei- ther outside nor inside had anything occurred to interfere with the regular action, which the various structures were designed to accommodate and promote. Among the enumerated causes of injury to the Roman aqueducts, it may be remem- bered, was the formation on the bottom and sides of the channel way, of a stony concretion, produced by matter deposited by the water. It is therefore satisfactory to know, what in- deed might a priori have been anticipated from the purity of the sources whence the Croton is fed, and the granitic region through which it passes, that