There are 4 cables of l0 inches diameter, each composed of 3640 wires of small No. 9 guage (sic), 60 wires forming one square inch of solid section, making the solid section of each cable 60.40 square inches, wrapping not included. * [* The wire was made of the finest quality of iron, and each piece was carefully tested before leaving the manufactory.]

The construction of these massive cables required extensive and somewhat complicated arrangements. The patent machinery for the transmission of wires across rivers was employed. This process, now so well known, was carried on for two seasons. I will therefore confine my remarks to the principles of this operation only, for the purpose of showing how a uniform tension and perfect work was insured. The appearance of the cables is not only pleasing, but their massive proportions are also well calculated to inspire confidence in their strength. The reflecting man, however, will naturally inquire: is this mass of wire put together so that the different strands bear all alike? Does each individual wire perform its duty, so that, when exposed to a great strain, they will resist with united strength to the last? This question can be answered in the affirmative; and that the tension of these 3640 wires composing each cable is so nearly uniform, that I feel justified in using the term perfect. The following remarks will explain more fully.

Each of the four large cables is composed of seven smaller ones, which are called strands. Each strand contains 520 wires. One of these forms the centre, the six others are placed around it. The 520 wires composing one strand are in fact one endless wire, obtained by splicing a number of single wires. The ends of strands are passed around and confined in cast-iron shoes, which also receive the wrought-iron pin that forms a connection with the anchor chains.

The strands were manufactured nearly in the same position which they now occupy in the cables, with about one-third of their present deflection; this was for the purpose of increasing the tension of the wires, and to facilitate their adjustment. *

[* It is an important fact that iron of a suitable quality for wire is increased in strength or tensile power nearly threefold by being drawn into wire. Thus a rod of iron capable of sustaining say 30 cwts. when drawn into wire of only one-third its sectional area, or, what is the same thing, into three times the length of the rod, will still sustain a load of 30 cwts. The expense of drawing the rod into wire bears only a small proportion to the increased length which is obtained. The expense of drawing wire to one-third its original size averages about 30 per cent. upon the cost of the rod iron, while its length is increased 300 per cent. This it is which has led to the extensive adoption of wire rope for collieries, railway inclines, and, still more recently, for the standing rigging of ships, in all of which there is great economy over the use of chains and hemp ropes. The advantage of using wire ropes instead of hemp ropes wherever they can be adopted, consists chiefly in their greater lightness and cheapness. A hemp rope capable of sustaining a given weight will weigh fully two and a half times more than a well made wire rope of the same strength. In the manufacture of wire ropes it is of the first importance that the wires be so laid together as that each individual wire shall bear its proportionate share of the load. Messrs. Newall & Co., of Gateshead, invented and patented machinery which perfectly accomplished this object. Their patent has, we understand, recently expired.-ED.]

All the preparatory operations, as oiling, straightening, splicing, and reeling, were carried on in an extensive shed, erected on the Canada side, back of the anchorage.-The mode of splicing has been frequently witnessed, and it has been noticed that the wire will break at any other point before it gives way at the splice. Fourteen large reels were constantly kept supplied with wire, ready prepared or spliced for going into the cables. The machinery for plying the wires across the river was worked by horse-power. The adjustment of the wires in the centre of the span was intrusted to two intelligent workmen, who were stationed on a platform, suspended by four wire ropes, about forty feet above the upper floor. Communicating all orders by means of signal flags, this whole operation went on very satisfactorily, occasional interruptions from high winds excepted. Owing to the influence of the sun, and the sudden changes of temperature of the wires on the opposite sides of a strand during the progress of its manufacture, great care was required on the part of the men stationed in tile centre. These and other circumstances have all been properly attended to; and there is every confidence that any difference of tension that may exist, does not exceed a few pounds per wire. The tension of one complete strand was about 50 tons, or 200lbs. per single wire. Two strands were made at the same time, one for each of the two cables under process of construction. On the completion of one set, temporary wire bands were laid on, about nine inches apart, for the purpose of keeping the wires closely united, and securing their relative position. They were then lowered to occupy their permanent position in the cables. On completion of the seven pair of strands, two platform carriages were mounted upon the cables, for laying on a continuous wrapping by means of the patent wrapping machines. During this process, the whole mass of wire was again saturated with oil and paint, which, together with the wrapping, will protect them effectually against all oxidation.

Five hundred tons of the wire used in the cables were manufactured by Richard Johnson and Brother, of Manchester, in England, and contracted for by Mr. James Cocker and Co., of New York. It is but justice to these parties to state here, that they have faithfully observed all the stipulations the contract imposed upon them. The specification required (see Appendix E.) that the wire, when suspended between two posts 400 feet apart, should not break at a greater deflection than 9 inches; also, that it should stand bending square over the jaws of a large pair of pliers, and rebending without rupture.-The size of wire was to be 20 feet per pound, but subsequently modified to 18 feet. The above test of strength corresponds to a tension of 1300 pounds per single wire, measuring 20 feet per pound, or to 90,000 lbs. per square inch of section. The contractors submitted a number of skeins for testing, which were all accepted. They then secured sufficient stock of the same quality of iron to fill the whole order, and were thus enabled to insure a uniform quality throughout. On delivery, the tests were continued with the same favourable results. From a great number of tests, which varied but slightly, I found the average deflection at which rupture took place, to be 0.683 feet, or a little over 8 inches. The wire measures 18.31 feet per pound, and the above strength, therefore, is equivalent to 16401bs. per single wire, or nearly 100,000 lbs. per square inch. By this mode of testing the wire is sure to give way at the weakest point. The above result, therefore, shows a remarkable uniformity in the iron, and great care in the manufacture of the wire.

Assuming the above average strength, the aggregate strength of the 14,560 wires composing the four cables will be 23,878,400 pounds. But their actual strength is greater, because the above calculations are based upon a minimum strength of the individual wires. The weak points of the different skeins will not happen to meet all at the same point. Being closely, and very compactly bound together, they will greatly assist each other. It is, therefore, safe in estimating the strength of the cables beyond the result of the above calculation. We may assume their aggregate ultimate strength at 12,000 tons, of 2,000 pounds each.

Next to severe strains, repeated vibrations and concussions of great intensity prove the greatest source of destruction to all kinds of metal. The more uniform and dense the iron is in its grain or fibre, the greater will be its durability. Good wire is a very safe and reliable material where great strains and vibrations are to be supported. Wire rope on inclined planes, where it is exposed to severe usage, and to an almost incalculable amount of vibration, lasts but a limited time. Its durability, however, will be found in direct proportion to the speed of its working, and to the consequent degree of vibration. Wire ropes of 1 1/4 inch diameter, on such inclined planes as those of the Alleghany (sic) Portage, in Pennsylvania, where there is a speed maintained of seven to twelve miles an hour, and where the machinery is very imperfect, and always out of repair, will not last longer than one and a half to two years, and will pass about 300,000 tons, gross weight, over planes of half a mile in length, and rising one in ten. Ropes of less size will perform five times the business on the planes of the Pennsylvania Coal Company, and on the inclines of the Carbondale road, because the treatment and machinery are so much better. Those in use on the inclined planes of the Morris Canal are 2 inches diameter; draw loads of 100 tons over inclinations of one in twelve, at a speed of five miles an hour, and last, in consequence of perfect machinery and good usage, seven to eight years. These facts are mentioned to show conclusively that the durability of wire rope and cables is in proportion to usage. The same rope will last much longer under a heavy strain movie', slowly, than it will under a light strain moving faster. Of this fact I have the most ample evidence. The experience is cited of wire ropes on inclined planes as an extreme, and by way of contrast. Suspension bridges should be built, so as to be entirely, or very nearly exempt from vibrations. The cables and suspenders of the Niagara Bridge are sustaining but a moderate tension; far within their elastic limits, and may be considered as at rest. They are also well protected against oxidation, and will consequently last an indefinite length of time.

In connection with this subject I will cite another interesting fact. The small cables which supported the temporary bridge put up under the superintendence of Mr. Ellet, and afterwards strengthened by Mr. Buchanan, had been exposed occasionally to heavy strains and to great vibrations, The wire originally was very good; about the same quality as that in the new cables, and made by the same manufacturer. On removal of the old work it was tested, and its strength and toughness scarcely impaired; so little, indeed, that I did not hesitate to work it into the new cables. Another feet is worthy of notice. The old cable measured 1 1/4 to 2 inches in diameter, and had been rapped at intervals of about 9 inches. The wire had been originally well coated with linseed oil, and the cables afterwards repeatedly painted with Spanish-brown or linseed oil on the outside, which made them impervious to water. On taking them apart I found the oil inside still in a soft condition, forming a tenacious varnish, and no trace of oxidation. These cables were put up in 1848 and removed in 1854; consequently had served six years. It is difficult to state how long this wire would have proved safe if it had remained in the same situation, exposed to the same usage. The wire suspension bridge at Friburgh, in Switzerland, the largest span in Europe, is still considered a safe work. It was completed about 1830. The roadway of the bridge is 808 feet long; weighs about 300 tons, and is supported by eight cables of 5 1/2 inches in diameter, containing in all about 4700 wires, No. 10. Its comparative strength is, therefore, much less than that of the Niagara Bridge, whilst it is frequently exposed to severe gales, and not secured against oscillations.

Wire cables, if guarded against oscillations, and not exposed to an undue tension, may be looked upon as of indefinite durability. I have cited wire rope on inclined planes as an extreme fact regarding durability. Severe friction, short bending, constant vibration, high tension, and frequent severe shocks, will soon wear out the best material. The more we can reduce these exposures, the greater will be its durability. The conditions of durability are certainly most favourable to the cables of the Niagara Bridge. An instance of comparative great durability is furnished by wire sofa springs, which, when made of good material, will not lose their elasticity under fair usage in a life-time. As another very remarkable case of great durability under the most severe exposure, we may refer to the wire strings of a piano, which are kept at a high tension, and in that state exposed to an almost incalculable amount of vibration. Common wire would not resist this action twenty-four hours. Piano wire is therefore made either of the best steel, or of bars which form a good steel outside, and a fibrous iron inside, purposely manufactured. Good piano wire furnishes a very remarkable instance, how much strength, and what a degree of elasticity can be obtained, by an improved quality of iron and steel. In this connection I may also point to the great durability of steel springs used for the support of carriages and railroad cars. Their great exposure to severe vibrations and constant concussions is well known; as also, their great durability. In all such cases of extreme service it has been well observed that the safe limit of elasticity is not exceeded, else the material will soon be destroyed. Bridges of half a mile span for common or railway travel may be built, using iron wire for the cables, with entire safety. But by substituting the best quality of steel wire we may nearly double the span, and afford the same degree of security.