The success of this extraordinary bridge is now to be considered an established fact. The trains of the New York Central, and of the Great Western Railroad in Canada, have been crossing regularly since the 18th of March, averaging over thirty trips per day.

One single observation of the passage of a train over the Niagara Bridge, will convince the most sceptical, that the practicability of suspended railway bridges, so much doubted heretofore, has been successfully demonstrated.

The practicability of suspended railway bridges of large spans, was a practical question of great importance to this peculiar country, intersected as it is by numerous large rivers and deep gorges, at a depression far below the general surface of the surrounding country.

The free and unobstructed navigation of the great rivers in the United States, which are to be crossed by railways, also demanded a new class of viaducts, such as will safely pass the Locomotive with its train at one bound, and at an elevation, that will leave no obstruction to the sailing and steaming craft below. The great rivers of this continent will no longer offer an insurmountable obstruction to the formation of uninterrupted lines of railways. At the completion of the first road to the Pacific we shall possess continuous lines of rail of over 3000 miles extent, over which, if desirable, cars loaded with treasure at San Francisco, may be passed to New York without breaking bulk.

The subject of suspension railway bridges was indeed a question of great importance. A railroad is now being constructed through the central part of the state of Kentucky, known as the Lexington and Danville line, which, with its extension to the state line of Tennessee, will form the connecting link between two great networks of railways, north and south, of such an immense extent as can only be found on the North American continent. This important connection will have to be abandoned, if a suspension railway bridge of a single span of 1224 feet, now in course of construction, across the Kentucky river, which there forms an abrupt chasm of 300 feet deep, cannot be accomplished.

The Kentucky river, the Niagara, and many others which have been ploughing their courses through limestone formations, will not admit of any other mode of crossing but by a suspension bridge. Tubular as well as arch and truss bridges are in those localities impracticable.

While the European engineers are engaged in the construction of short lines of railways, at such enormous cost that in most cases the capital invested yields no remunerative dividends, the task of the American engineer is to lay down thousands of miles with extensive bridging, at a cost which would barely suffice in Great Britain to cover the expenses of preliminary proceedings.

The work which I had the honour to have entrusted to my charge has cost less than 400,000 dollars,-less than £80,000. The same object accomplished in Europe would have cost one million of pounds, without serving a better purpose or insuring greater safety. The mixed application of timber and iron in connection with wire, renders it possible to put up so large a work at so small a cost. When hereafter, by reason of greater wealth and increased traffic, we can afford to expend more on such public works, we shall construct them entirely of iron, omitting all perishable materials. We may then see railway bridges suspended of 2000 feet span, which will admit of the passage of trains at the highest speed.

As regards the success of this great work, more has been accomplished than was promised. The idea of a perfectly rigid structure, such as a tubular bridge, was never held out. The Niagara Bridge possesses all the stiffness that is wanted, and much more than is actually needed for the safe passage of trains. It is gratifying to notice the entire absence of all such vibrations as would easily be noticed, or would eventually prove a source of destruction. There is no bridge in the world, neither of stone, cast or wrought iron, which is free from all vibrations. The effect of the concussions of a fast moving train may be sensibly felt miles off through the solid earth, while buildings of brick in the immediate vicinity of a railroad are very perceptibly shaken. Sitting upon a saddle on top of one of the towers of the Niagara Bridge during the passage of a train, moving at the rate of five miles an hour, I feel less vibration than I do in my brick dwelling at Trenton, N. J., during the rapid transit of an express train over the New Jersey railroad, which passes my door within a distance of 200 feet. I will further remark that the land cables are not at all affected by the passage of trains; the very slight vibrations and concussions noticeable in the superstructure are not transmitted over the towers. This fact is gratifying, as it will insure the durability of the masonry. The stiffness of the lower floor teas been a matter of general observation ever since its opening. Strange as it may appear, a number of loaded teams produce more motion than results from the transit of a train. But for the rumbling noise over head, such transit would not be noticed by persons on the lower floor.

Suspension bridges have generally been looked upon as loose fabrics swung up in the air, as if for the very purpose of swinging. Repeated failures of such works have strengthened this belief. My success in the construction of suspended aqueducts, however, should have been deemed a strong argument against it, at least by professional men. This fact should have cautioned them against forming hasty conclusions upon a subject which they had but partially investigated. I have built five such works, and two of them of large capacity and great extent, which have all proved successful, and are to all intents and purposes as rigid as stone or cast iron aqueducts. The principle of suspension is certainly much easier applied to aqueducts than to railway bridges; but still these works require a degree of solidity and stiffness, which, as was at the time reasoned by the profession generally, could not be obtained. But some nonprofessional men saw much clearer than professional men, and so the thing was done. A good deal of the same sort of reasoning which was made use of against aqueducts' was equally directed against railway bridges, but with no better success, as the result shows.

Professional and public opinion having been adverse to suspended railway bridges, the question now turns up' what means have been used in the Niagara Bridge to make it answer for railway traffic? The means employed are: Weight, girders, trusses, and stays. With these any degree of stiffness can be insured, to resist either the action of trains or the violence of storms, or even hurricanes; and in any locality, no matter whether there is a chance of applying stays from below or not. And I will here observe, that no suspension bridge is safe without some of these appliances. The catalogue of disastrous failures is now large enough to warn against light fabrics, suspended to be blown down, as it were, in defiance of the elements. A number of such fairy creations are still hovering about the country, only waiting for a rough blow to be demolished.

Weight is a most essential condition where stiffness is a great object, provided it is properly used in connection with other means. If relied upon alone, as was the case in the plan of the Wheeling Bridge (U. S.), it may become the very means of its destruction. That bridge was destroyed by the momentum acquired by its own dead weight, when swayed up and down by the force of the wind. The weight of a suspension bridge should not bear too small a proportion to the transient loads it is calculated to support. The smaller the transient weight is in proportion to the weight of the structure, the less disturbance such passing loads will cause in its equilibrium. When a train enters the Niagara Bridge it produces a slight depression upon that part of it. But this depression cannot take place without a corresponding rise at the opposite end. The greater therefore the weight of the structure, the less its equilibrium will be affected by transient weights. This is certainly plain, and will appear so to the most careless observer. Consequently, a high wind, acting upon a suspended floor, devoid of inherit stiffness, will produce a series of undulations, which will be corresponding from the centre each way. And from this follows the necessity of introducing the principle of the triangle, so as to form stationary points and thus check vibrations and restore balance. The effect of trains has to be met in the same way by the application of the triangle, either in the form of stays or trusses, or both. Undulations caused by wind will increase to a certain extent by their own effect, until by a steadyblow a momentum of force may be produced, that may prove stronger than the cables. And although the weight of a floor is a very essential element of resistance to high winds, it should not be left to itself ta work its own destruction. Weight should be, simply, an attending element to a still more important condition, viz.: stiffness. Before enlarging upon the subject I will here remark, that an engine and tender of 34 tons weight, together with one passenger car, crowded with persons' making a total load of about 47 tons, caused a depression in the centre of 51 inches. This flattening of the camber is partly owing to an actual elongation of the cables, and in part to the disturbance of the equilibrium, the weight in the centre causing the ends next the towers to rise. But suppose the superstructure to possess no inherit stiffness at all, and together with the cables to be perfectly flexible, then the depression caused by the above weight would be much greater.

Let x represent the deflection of cables in feet.

w " the weight in the centre in tons.

and W " the weight of the whole structure, then the formula x w / 2 W will give the depression produced in the centre in feet. Substituting now 59 feet for x, 47 for w, and 1000 for W, we shall have 59 x 47 / 2 x 1000 = 1,386 feet.

But the actual depression, as observed by an instrument, was only 52 inches or 0.45 feet. The difference of 0 936 feet, therefore, is owing to the inherent stiffness of the structure. A single engine of 23 tons weight, including tender, caused a flattening of the camber in the centre of 0.3 feet. The formula applied to this case would give a depression of 0.678 feet, or 0.378 feet more. The above formula, however, does only consider the equilibrium of the catenary. Neither the elongation of the cables, nor the movement of saddles on the towers, nor the reduction of deflection of the land cables are taken into account. To provide for all these movements, would exceedingly complicate the construction of a formula. The importance of weight however is rendered manifest, because it is shown that depressions are indirectly as the weight of the structure, and directly as the weight of the transient load; also, directly as the deflection of the cables. The flatter the cables are the stiffer they will be, but also less able to support a load.

In depending upon weight as guarding against a disturbance of equilibrium, such element should also serve to increase the stiffness of the structure mechanically and not only statically. I object to weight put on simply as loose weight.

In those discussions which took place in Great Britain on the subject of suspension bridges previous to the adoption of the tubular plan for crossing the Menai Straits, a more thorough investigation of the subject would have led to the conclusion that there is no inherent defect in the suspension principle, and that by simply adding to its weight, without providing any other means of stiffness, its adeptness to railway traffic would have become clear.

The idea of absolute rigidity must be abandoned, when considering the practicability of suspension railway bridges. We can only obtain a comparative degree of rigidity in any kind of structure, no matter whether it is a stone or cast-iron arch, iron or wooden truss, or a hollow wrought-iron beam. Such being the case, the next question is: What degree of rigidity is necessary for the safe passage of trains at certain speeds? Flexibility in a bridge is no objection, provided it offers no obstruction to its use, and is compatible with safety and durability. The Conway tube of 400 feet span deflects 3 inches, under a weight of 300 tons placed in the centre. How much would a tube of 800 feet span deflect under the same load, provided such tube had the requisite depth and strength ? Probably no less than 9 inches. When the Niagara Bridge is loaded with a freight train, covering its whole length, and weighing about 326 tons, the camber is reduced 0.82 feet, or nearly 10 inches. On removal of the load the structure rises again to its former level. In the case of the Conway tube, the deflection is owing to the elasticity of the iron plates composing it. In the Niagara Bridge the same cause produces the same effect, but in different directions, and under different circumstances. In the tube one portion of the iron is exposed to tension, while a greater portion is exposed to compression. But the tensil power of wrought-iron is much greater than its resistance to compression. In a suspension bridge, on the other hand, nothing but the tensil force of wrought-iron, of a form and size which insures the best quality, is employed. The tubular principle involves a great waste of material when compared to the suspension principle, and consequently, whenever great weights are to be supported over large spans, the first cannot successfully compete with the latter. In a country where the engineer's task is to make the most out of the least, the suspension principle will henceforth take the lead of the tubular in all ordinary localities. For extraordinary long spans the tube cannot compete on any terms.

Every train that passes over the Niagara Bridge causes a certain depression, but this being far within the safe limits of the elasticity of wire no injury results from it. Every train that passes through the Conway, or through the Britannia tubes, causes a depression. Now, can it be said that this deflection is an objection to the tubular principle? Certainly not, because these deflections are far within the safe limits of the elasticity of the iron plates composing them. But tubular bridges are designed to be rigid, while suspension railway bridges are designed to be flexible.

Next to weight as a means of preserving equilibrium, the most important feature in the Niagara Bridge is the girders which support the track. They are made of timber and in connection with four lines of rails serve to distribute the pressure of concentrated loads. The efficiency of these girders became evident at the first trial. On the 8th of March I made the first trial trip with an American built engine of 23 tons weight, with four drivers placed but a short distance apart. The general depression in the centre was 0.3 feet. But its passage was also accompanied by a local depression or slight flattening effect, which amounted to about 1 inch extending over a length of 100 feet. Another American engine of 22 tons weight produced nearly the same effect. I then made a trip with an English built freight engine of 34 tons weight, with six drivers, placed at a considerable distance apart, which, owing to its weight being less concentrated, did not cause more of a local deflection than half an inch, but together with a loaded passenger car produced a general reduction of the camber in the centre of 5 1/2 inches. Without girders the trusses would not long resist the action of trains.

The Niagara Bridge, of a span of 821 feet 4 inches from centre to centre of towers, forms a slightly curved hollow beam or box of a depth of 18 feet, width of bottom of 24 feet, and of top 25 feet. The lower floor is used for common travel, while the upper is appropriated to railway business and sidewalks. The two floors are connected by two trusses of a simple construction, so arranged that its resisting action operates both ways, up as well as down. The suspenders are 5 feet apart. The beams of the upper and lower floor are connected by posts arranged in pairs, leaving a space between for the admission of the truss rods. The ends of the posts are secured between the beams in a manner that no part is weakened, and that any amount of strain can be thrown upon them without injuring or loosening their connections. There are no joints to work loose. If the timber should undergo a further shrinkage, the truss rods will simply require tightening. The depressing action of any loads is by these posts transmitted from one floor to the other. From the end of each pair of posts, a truss rod extends each way to the fourth pair of posts at an angle of 45 degrees. The rods therefore cross each other and form a diamond work. They are 1 inch diameter, their screw ends 1 1/8 inch. The pressure upon any pair of posts is by these rods spread 40 feet apart. The nut work on cast-iron plates is placed above or below the posts.

Without adding much to the weight of the structure, a surprising degree of stiffness has been obtained by the united action of the girders and trusses. They have fully realized every expectation. The pressure of an engine and of a whole train of cars is so much distributed, that the depression caused by a light freight or ordinary passenger train is not readily observed. A freight train of twelve loaded cars with a 25 ton engine, covers little more than half the length of the floor. Its effect is more marked and noticed than either a smaller or larger train. When in the centre the result is only a flattening of the camber, hut when near the towers where the grade forms nearly a straight line, the depression is from 3 to 4 inches. A longer train of greater weight in proportion disturbs the equilibrium less, as it covers a greater extent. Passenger trains of fifteen long cars, which frequently cross the bridge, make little impression observable by the eye. While the severe action of trains upon common arch and truss bridges causes great wear and tear, I am persuaded that the woodwork of the Niagara Bridge will suffer much less. My observations during the last month have not caused me to change this view, which I have always expressed.

The tubular or box plan of the bridge has added much to its stiffness, vertically as well as horizontally. There is an entire freedom from all lateral motions during the passage of a train. It is a surprising fact, that half a dozen heavy teams on the lower floor produce a more perceptible horizontal motion, and a much greater jar and trembling than is caused by a train of cars moving at the stipulated speed of five miles an hour. The smoothness, evenness, and perfect level condition of the railroad tracks partly accounts for this. While teams on the lower floor generally move forward outside of the centre of the bridge, the trains are exactly poised in the centre. The great horizontal stability of the work is mainly owing to the powerful lateral bracing of the upper cables, which are suspended in a very considerable inclination. There is no reason to suppose that the durability of the woodwork of this bridge will be less than that of a common suspension bridge, serving for ordinary travel alone.

The next means of stiffness I have applied are stays, above as well as below the floors. These, as well as the suspenders, are all made of wire rope, manufactured at my works at Trenton, N.J. There are 64 diagonal stays, of 1 3/8 inch diameter rope, above the floors, equally distributed among the 4 cables. They are fastened to the suspenders by small wrappings, so as to form straight lines. Each of these stays represents the hypothenuse (sic) of a rectangular triangle, of which the two cadets are formed by the towers and the floors. These two being solid and rigid in the direction of the lines they represent, by preserving the straight line of the stay, and not allowing them to sag or deflect, we form as many triangles as we have stays. Now the triangle is the only geometrical figure whose corners cannot be shifted, consequently, by keeping those stays under a good tension, we form so many stationary points in the flooring as we have stays. But these stays do not only stiffen, they are also a great assistance to the cables. Their number being limited, and the cables possessing an abundance of strength, I did not continue them over the towers to the anchorage. They are secured to the saddles, and allowed to move with them. No fear need be entertained that they will pull the saddles forward. The friction of the cables in the saddles is, at the lowest estimate, equal to one-third of the pressure. The constant pressure upon each tower is 500 tons. This would give 166 tons. The ordinary tension of each stay being about 4 tons, the united horizontal force of 16 stays applied to 2 saddles is found to be about 56 tons, to which a resistance of 166 tons is opposed, without taking into account the curvature of the cables in the saddles, which will nearly double it.

To the underside of the lower floor 56 stays are attached, which are anchored in the rocks below, and occupy positions calculated to insure against horizontal as well as vertical motions. Their principal duty is to guard against the force of winds, but at the same time they contribute materially to preserve the equilibrium of the structure during the passage of trains. Their usual tension averages from about 2 to 3 tons. Considering their positions, their aggregate force, exerted upon the lower floor in a vertical direction, at a medium temperature, is less than 100 tons. In summer this force is less, in winter it is more. In the disposition of these stays, I have takers advantage of the ample opportunity this locality offers. There are bridge sites where this cannot be done, and where security against the force of winds has to be entirely obtained by over floor stays, and by the inherent stiffness of the structure. But the difficulty is no greater in the one case than in the other. In all localities perfect safety against the force of winds can be obtained.

To present a fuller analysis of the work, I will review its various parts in the same course in which they were put up, and commence with the anchorage.