The writer of the following report is the projector of a number of extensive and important public improvements which have attracted much attention, one of which, the Broadway Arcade Railway, is well known to our readers. According to Mr. Nowlan's figuring the proposed East River Suspension Bridge, although the plans are indorsed, either tacitly or expressly by nearly all our leading engineers, will be a dead failure. He thinks it cannot be made to hold together except for a short time, and that with the hight of towers proposed the bridge will almost touch the surface of the water at high tide. Mr. Nowlan's report contains several interesting statements, and we have no doubt will call out suitable replies. It is, we believe, the first adverse report upon the project that has been made public : Report on the construction of suspension bridges over the East River as proposed by a company incorporated by the Legislature of the State of New York, made before the Commissioners appointed under an order of the Senate of the United States, to meet at the city of New York, to hear such objections and recommendations upon the subject of such, bridges as may be made by competent persons, professional or otherwise, such commission consisting of Gen. Newton, Gen. Wright, and Major King, all of the United States Army. EBPOBT OF SAMUEL BASSES B. NOWJAN, C. E. QenUemen: In reply to your request, I submit the following report, based upon an experience of many years in practical engineering, and the attendant scientific investigation of details, particularly as applied to engineering manipula* tions in the construction of military works in connection with submarine engineering. The proposed bridge, according to the plans now before the Commissioners, will be very nearly one mile (5,228 feet) in length. The abutment on the New York side will be at pier No. 29, and on the Brooklyn side at the slip at Fulton Ferry, The grade on the approach from the New York terminus at the City Hall Park to the level of the bridge will be 3- feet in every 100 feet, while the grade on the Brooklyn side, from its terminus near the junction of Sands and Fulton streets, will be less. The hight of the bridge is to be 135 feet, as fixed by the State charter. The center span will be 1,600 feet. It is very doubtful if 135 feet of hight would be sufficient to allow the passage of vessels of a large tunnage, and it seems impracticable to increase the hight of the bridge by reason of the steeper grade, which would render it too great fortheconvenience of travel. In slippery weather wagons would find it impracticable to ascend to the elevation of even 135 feet, and passengers would prefer the ferry boat. As to the proposition of any bridge on the suspension plan by wire cables or iron chains, I desire particularly to give the causes and practicable results in cases of failure under similar circumstances. Referring to the diagram, I would remark that tho distance spanned will range about 1,600 feet. A B represents the chord, A C B the catenary curve with the line C D. Now, as the natural sag of the suspended chain should be in proportion as 1 is to 16, and the towers being as represented, 135 feet at the point of hight for the chord, the catenary curve being 1 in 16 would produce in the distance of 1,600 feej; a sag of 100 feet, leaving only 35 feet for water way, Should an unnatural strain or taut be brought to bear upon the suspended chain it would not allow for the deflection and variations of temperature, which from extraordinary changes may vary from 120 Fah. to 20 below zero. When the catenary curve is obtained, a natural curve is obtained which will meet all deviations of temperature. But if not, the overstrain or taut will cause the snapping under the vibration, as in the ense of the Menai Suspension Bridge. The cause of the falling of that bridge was from the oscillating motion to which it was subjected, there being no strands employed on that bridge as now used by the projectors of the Niagara Suspension Bridge. If those strands were not used that bridge would not last half its time. At present the deflection is over 9 inches at noon under a temperature oi 85. At the time it was first built it gave only 5 inches on the catenary curve. The great feature of the suspension bridge over the Bast River will be the two towers, and as the grade of the approach is given at Si feet for 100, and the towers are to be 135 feet, this will give the hypothetical grade line of 3,717 feet, which ?will carry the roadway across Broadway if the line be to the City Hall Park, or if it be taken in the direction of the Bowery will reach about Chatham Square. Now, if we consider the immense expense of some $5,000,000 or $6,000,000 merely to make an approach, without including the cost of construction, we may appreciate the motive that would induce such an unnecessary outlay of public money. Each of these towers is proposed to be 134 feet at its greatest axis on line at right angles with the thread of the stream, and 36 feet tha lesser axis in line with the thread of the stream; below the upper cornice at top of the tower these dimensions are reduced to 120. The elevation of the flooi will be 118 feet above high water mark. The roofing above the floor is 150 feet, which will be a total hight of 263 feet from high water mark to this proposed roof. And the commencement of each tower will be three feet below low water mark, with a cubical content of stone in the two towers of 62,824 yards of 27 cubic feet each. The cubical contents of one tower 31,412 cubic yards multiplied by 27 cubic feet will give 848,124 cubic feet, or 67,850 tuns; add to this the great est weight of superstructure and load of 4,753 tuns, and it gives a total of 72,603 tuns. Now, the area of base at low water line is 4,660 feet, and therefore the pressure of the structure on each superficial foot will be 15 58 tuns. The admitted usual pressure on the superficial foot is from 3 to 4 tuns on all railroad engineering in the construction of railroad bridges on piers of 50 or 60 feet in hight, and such pressure is always deemed secure on a bed of compact gravel or sand, provided there ia no danger of undermining or spread ing laterally. This great weight sustained between these two towers, 1,600 feet apart, and sustaining a compound leverage and lateral abutting power, will evidently increase the destructive action of the dead weight of gravity of 4 chains. The weight given of the superstructure without cables will be 2,675 tuns, stretched over a space of 1,600 feet, the leverage strain on the center will be as 1 is to 8 by progression, so that 1 tun in distance from the abutting point is increased in its gravity to 8 tuns in the proportion of 1 foot to 8 feet. By this calculate at half of 1,600 feet what is the supporting power required to support a dead weight of half the mean weight of superstructure, 4,753 tuns, equal to 2,376 tuns, which, by the accumulated strain on the spandrel, as we may express it, of one side of an arch which is to cross a space of 1,600 feot, this will give a distance to each spandrel oi 800 feet, and as each tun actually requires from every abutment for every 8 feet just 8 times its own weight to suspend it, then, consequently, 16 feet leverage will require 16 tuns abutting force to support 1 tun at that distance, and so on in proportion for the length of power represented and required at the extreme end of the entire section of each chain at mid center representing these spandrels. When it is borne in mind that the action of the temperature varies from 10 below zero, to 120, Pah., the destruction of this chain is greatly increased by the vibration in stormy weather, under a taut strain, below zero, and the deflection of the chain under a high degree of summer heat; and as the sagging of the chain on the catenary curve, can never of itself return to the original at night, say 60 which at noon gave 95, Pah. The dead weight, has no power to rise of itself the space of its noon deflection, as illustrated in the Niagara Suspension Bridge, which when first constructed deflected only five inches on the span of 800 feet. But the set strain at each deflection, has now caused the present Suspension Bridge to permanently sag four inches from its former constructed catenary curve, the sine line being increased to 9 inches at noon, at a temperature of 95 Pah. This accumulating set strain caused the breaking of the suspension bridge at Brighton, England, and the Milford suspension for railway transit, which broke down in 1832, and 250 lives were lost, the train falling a distance of 46 feet into the river. This principle of suspension bridges has been superseded in Europe by the tubular bridge invented by the celebrated Engineer Stephenson, who constructed the Victoria Bridge overtlie river St. Lawrence, Canada. In the plans I submitted to the Commissioners, I obviate all these detects, and can construct a permanent bridge with arches indestructible, of 500 feet span, and 200 feet above high water mark, with a permanent approach through fireproof iron buildings, on line with the thread of the stream, forming stupendious and magnificent bonded warehouses capable of paying the entire expense of each building, in sixteen years, and without using one foot of private property, with free access to the river front at all points, as these constructions are raised 25 feet above the sidewalk and the flooring forms a perfect shelter for all merchandise temporarily wharved, from rain or snow in winter, and from the sun in the heat of summer. I have the honor to "be, Gentlemen, respectfully yours, SAMUEL B. B. NOWLAN, C. E. NEW YORK, April 17,1869. MARYLAND INSTITUTE EXHIBITION.—The second annual exhibition of the Maryland Institute for the promotion of the Mechanic Arts, will be found advertised in another column. The first was a success, and doubtless the second will also be a fine display. Manufacturers and inventors will do well to notice. MAGNETS, whose coils are long) discharge their magnetism much less easily and slowly than those whose coils are short.