plenty and capital scarce; and where improvements must necessarily be of a more temporary character.

With this view of the subject, I have given considerable attention to the details of wooden bridges; and, with a good deal of investigation and experiment, have arranged plans which are confidently believed to possess important advantages over the plans generally in

use.

The preceding few pages have been transcribed from the author's original and first essay upon bridge building; and are introduced here, not on account of any practical value they may possess in the present state of progress in the science of bridge construction. But they may possess some little interest as marking about the starting point of the construction and use of Iron Truss Bridges.

If the estimates above exhibited, of the cost of iron bridges, appear small and inadequate, under the lights furnished by the experience of a quarter of a century, much allowance may be claimed on account of the change of times and circumstances within the period in question. And, when it is borne in mind that the author actually contracted for, and built iron railroad bridges of 40 and 50 feet span, for $10, and of 146 feet for $30 per foot, the estimates above given may not seem entirely preposterous, although much higher prices are obtained for bridges of like dimensions at the present day.

PRACTICAL DETAILS.

LXXXV. In preceding pages I have endeavored to give a short and comprehensive general view of the

subject, and to ascertain and point out the best general plans and proportions, for the main longitudinal trusses, or side frames of bridges, and the relative stresses of their several parts.

The side trusses may be regarded as vastly the most important parts of the structure, and the strength and sufficiency of these being secured, there is much less difficulty in arranging the remaining parts, the forces to which they are exposed being much less than those acting upon the trusses. I propose now to enter more into details, and give such practical explanations and specifications as to the strength of materials, the methods of connecting the several parts or pieces, both in the main trusses, and other parts of the structure, illustrated by the necessary plans and diagrams, as, it is hoped, will enable the young engineer and practical builder to proceed with judgment and confidence in this important branch of the profession.

IRON BRIDGES.

STRENGTH OF IRON.

LXXXVI. Iron has the power of resisting mechanical forces in several different ways. It may resist forces that tend to stretch it asunder, or forces which tend to compress and crush it; the former producing what is sometimes called a positive, and the latter, a negative strain. It may also be exposed to, and resist forces tending to produce rupture by extending one side of the piece, and compressing the opposite side; as where a bar of iron supported at the ends, is made to sustain a weight in the middle, which tends to stretch the

lower, and compress the upper part. This is called a lateral, or transverse strain.

Iron may likewise be acted upon by forces tending to force it asunder laterally, in the manner of the action of a pair of shears. This is called a shear strain ; and though less important than either of the preceding cases, it will frequently have place in bridge work, partially at least, in the action of rivets, and connecting pins.

With regard to the simple positive and negative strength of iron it is only necessary for me to state in this place, as the result of a multitude of experiments, that a bar of good wrought iron one inch square, will sustain a positive strain of about 60,000lbs. on the average; and a negative strain, in pieces not exceeding about twice the least diameter, of 70 or 80 thousand pounds. But in both cases, the metal yields permanently with much less stress than the amounts here indicated; and hence, as well as for other considerations, it can never be safely exposed in practice, to more than a small proportion of these stresses, say from to 4.

Cast iron resists a positive strain of 15,000 to 30,000lbs. to the square inch, but usually, not over 18,000. But it is seldom relied on to sustain this kind of action especially in bridge work, wrought iron being much better adapted to the purpose. On rare occasions, it may perhaps safely be exposed to a strain of 3,000 to 4,000lbs. to the square inch, but should not be used under tension strain, when wrought iron can be conveniently substituted.

Cast iron, however, is capable of resisting a much greater negative strain than wrought iron; its power of resistance in this respect, being from 80,000 to

140,000lbs.; seldom less than 100.000 to the square inch, in pieces not exceeding in length, twice the least diameter.

But in pieces of such dimensions as must frequently be employed in bridge work, fracture would take place by lateral deflection, under a much smaller force than what would crush the material. It is therefore necessary to take into account the length and diameter, as well as the cross-section, in order to determine the amount of compression which a piece of cast iron, or any other material may be relied on to sustain.

LXXXVII. The cause of lateral deflection resulting from forces applied at the ends, and tending to crush a long piece in the direction of its length, is supposed to be a want of uniformity in the material, and a want of such an adjust of the forces that the line joining the centres of pressure at the two ends, may pass through the centre of resistance in all parts of the piece.

These elements are liable to considerable variation, and can not be very closely estimated in any case. Therefore the absolute power of resistance for a piece of considerable length, can not be deduced by calculation from the simple positive and negative strength of the material, but resort must be had to direct experiment upon the subject; and, even wide discrepancies should naturally be expected in the results of experiment, unless the lengths of pieces experimented upon, be very considerable.

In respect to pieces, however, having their lengths equal to twenty or more times their diameters, a somewhat remarkable degree of uniformity is found in their powers of negative resistance, and the following formula, deduced theoretically, though not fully sustained

by experiment, may be useful in determining approximately the relative powers for pieces of similar crosssections, but different dimensions. The power of resistance (R), is as the cube of the diameter (d), directly and as the square of the length (), inversely, that is, R is as 7.

d

FIG. 25.

The reason of this formula may be illustrated with reference to Fig. 25, in which adb represents a post loaded at a, so as to bend it into a curve, of the half of which cd is the versed sine. It is obvious that in this condition, the convex side of the post is exposed to tension (or at least, to less compression than the other side), and the concave side to compression; also, that the effect of the load at a, toward breaking the post at d, is as the versed sine cd, which is as the square of ab. But the power of the post to resist rupture transversely, is manifestly as the cross-section of the post (i. e., as the square of the diameter), multiplied by the diameter. Hence, the power is as the cube of the diameter. Now, the ability of the post to sustain the load at a, is directly

d Hc

as the power to resist rupture, just determined, and inversely as the mechanical advantage with which the load acts, above seen to be as the square of the length of the post. Hence, the formula.

We shall see as we progress, the relation which this formula seems to bear to the results of experiment.

The following list of experiments made by the author some 25 years ago, though few in number, and upon a somewhat diminutive scale, nevertheless, may afford some light as to the law governing the resisting power of cast iron in pieces of different lengths, as compared