One pound of wire 1-8 inch thick, measures 20 feet, and if manufactured ofgood material, and of the right temper, will bear 1350 pounds, and more. Fromthis it follows, that a length of wire of 27,000 feet would support itself, whenfreely suspended. Let us further suppose a wire, in place of being suspended atone end, suspended at both ends to two points 17,000 feet apart, and so that thiswire will deflect 1400 feet, or about one twelfth of the distance, then the tensionresulting from its own weight, will be equal to its strength. Any wire, there-fore, freely suspended at both ends, within a distance of 3 miles, and with adeflection of about 1-12 of that distance, will bear its own weight without arupture. But in this case the material would be strained to its ultimate capacity.Experience has taught that iron should not be strained beyond the limit of itselasticity. By the term elasticity, is understood the capacity of iron to contractits fibres again after the strain ceases which caused their expansion. Tensionhas the same effect upon iron as heat, but only in the direction of the fibres.The elasticity of bar iron is limited to one-third of its breaking strain. A barof one inch square should, therefore, never be taxed with a greater weight than20,000 pounds, or 10 tons. The amount of elasticity of wire, when comparedto its ultimate strength, is much greater than of bar iron. Its degree of elas-ticity, as well as its ultimate strength, is likewise dependant upon its temper.

One-half of the weight which would break a wire, may be supported by itwith safety, provided this weight is at rest, and causes no vibrations. A wirecable, therefore, freely suspended over a space of 1 1-2 miles, and deflecting1-12 of this distance, will, with safety, support its own weight for any length oftime. If we contract this distance, and thereby reduce the weight of the cable,we shall enable it to bear some additional weight beside its own. If there wasany necessity for the construction of a span of 3000 feet, such a bridge couldbe constructed and rendered as safe as a span of 100 feet long. The practi-cability of such a work could not be doubted, it would only be a question ofexpense.

In determining the strength of cables, great allowance should be made forthe support of vibrations. The weight of a floor, for instance, if set in motionby a hurricane, would cause a greater strain than would result from the greatestload which the floor is capable of holding. This shows the necessity of a stiffconstruction, capable of resisting high winds, If properly constructed, a sus-pension bridge should support the effects of a hurricane, which would destroyall other kinds of wooden bridges, and prostrate trees and houses. The yield-ing nature of the structure would save it, provided its stiffness was great enoughto check undulatory vibrations.

The greatest strain which could be produced by a transitory load, would becaused by a body of soldiers marching over the roadway in compact file. Theleast allowance of room for one man in rank and file is 3 superficial feet. Theroadway being 20 feet 6 inches wide, would contain an area of 20 1-2 superficialfeet for every foot length. No marching could take place on the sidewalks; asthey are separated from the roadway, they would be used by the officers incommand. The average weight of a soldier, fully equipped, may be estimatedat 150 pounds, which would make 1050 pounds for every foot length of road-way. Marching of soldiers, however, should never be allowed on any bridge,and should be strictly prohibited. They are to be ordered out of step, andallowed to move on promiscuously over the roadway as well as the sidewalks.In a crowd of people, moving freely, 4 superficial feet of area and 120 poundsof weight, is about a fair average allowance for one person. The spacesbetween the railings, as far as the floor can be occupied, measures 32 superficialfeet for one foot length of floor. There would, therefore, be room enough for