be upset, so as to give sufficient width and strength at the bottom of the stirrup to allow a 1" stem to be screwed in, to pass through and support the connecting block. This stem may extend above the bottom of the stirrup, about ", a hole being made in the under side of the beam to receive that projection. The thread of the projecting part of the screw, which euters the beam, should be turned or chipped off. This plan may be used in bridges either with or without side walks.

Again, the upright may terminate in a flange at the top of the beam, and bolts screwed or cast in the top of the block, or running through the block with head or nut below, one on each side of the beam, and connecting with the flange of the upright, as shown at B, Fig. 32.

In the case of double uprights, the beam being cut to go between the inner branches, the fixture plates should lap about 20" upon the beam, and extend so as to clasp both branches of the upright.

CX. To introduce the solid wrought beam in bridges with sidewalks originally constructed for wooden beams, the following plan is suggested.

Let the beam be cut, say 1" shorter than the space between opposite uprights. Then, take for each end

of the beam, two plates" thick and 7" wide, or, wide enough to fill the space between the flanges of

the beam at 1" from the centre, so that one being placed on each side, they will be kept far enough apart to admit the upright between them. The plates should be long enough to lap 20′′ upon the beam, and extend to outside of side walk. They may be bolted with two 1" bolts near the end of the lap, and one near the end of the beam by the upright; as seen under the letter u in Fig. 33. A 14" bolt in the centre of depth, and 7 or 8 inches from the upright, will serve both to aid in holding the plates in place, and to connect the sway rods. These plates should not be cut by bolt or rivet holes in the upper part, except at considerable distance from the upright u.

FIG. 34.

Small bolts or rivets, r r, etc., should be inserted at intervals of 9 or 10 inches, near the lower edge, with thimbles to stay the extension plates apart, leaving a space equal to the diameter of the upright. In Fig. 33, s-w is a part of the extension for supporting side walk; s, a cast iron saddle weighing about 4lbs. for joist bearings, and c, a cross-section through the splice.

R

To afford a proper bearing upon the connecting block, it is proposed to use a wrought iron ring (R. Fig. 34), high

enough to throw the whole weight upon the extension plates ee, and " to 1" in width, except on the side next the beam proper, where it is to be clipped or drawn down to ". This, however, is not an essential point. In case of bridges already erected, the ring will have to be left open as at R', and when used, heated and closed around the pright.

CXI. The Link Chord, composed of a set of links to each panel, connected by pins or connecting blocke (the latter affording also points of attachment for ver ticals, diagonals, &c.), both for Arch and Trapezoidal trusses, was originally adopted by the author, as the readiest means of putting the requisite amount of chord material in a manageable form, both as it regards manu facturing the parts, and erecting the structure. This form renders the whole section available for sustaining tension, avoiding any loss in rivet or bolt holes for forming connections.

The experience of more than a quarter of a century, during which time many hundreds of bridges with link chords have been constructed, and used in almost all conceivable conditions, (in many cases, undoubtedly, the links having been but imperfectly manufactured and fitted to the connecting blocks), with a degree of success and satisfaction seldom exceeded, may reasona bly be regarded as fairly establishing the efficiency and safety of this mode of construction, when proper care is used in the performance of the work.

Continued and successful usage in a multitude of instances, is regarded as a better criterion as to the reliability of a plan of construction, than a small nuer of isolated tests, however severe; and such usage the link chord has been subjected to.

CXII. The theoretical questions to be considered in this case, would seem to be, as to the possible deterioration of the cohesive strength of the iron, produced in forming the bends at the ends of links the indirect, or lateral strain in those parts, resulting from imperfection of the fitting to the connecting block or pin, and, the imperfection of the weldings, both as it re

-

gards complete cohesion, and the tendency to crystallization under the welding heat, not being fully destroyed by subsequent hammering and working.

The whole process of the manufacture and refinement of iron, is based upon the principle that disconnected pieces of iron brought in contact under intense heat, but without complete fusion, and subjected to violent compression, as by hammering or rolling, will unite, and become a single piece or mass.

66

Every bar of refined iron found in the iron market, is composed of half a dozen or more parts, which were once separate and disconnected. Those having been fagoted," or placed in juxtaposition, and submitted to a welding heat, and passed repeatedly between ponderous rollers, or subjected to the blows of heavy hammers, are united and drawn into bars of required sizes and forms for use.

These masses, taken from the furnace and suffered to cool without hammering or rolling, would be found more or less crystaline and brittle. But the latter operations prevent such a result, and the iron becomes more or less soft and flexible, even in a cold state.

Iron which has undergone the uniform process of rolling, is generally of uniform quality and strength throughout the whole piece; and, as far as it can be used in that state, without re-heating and re-working, it may be regarded as somewhat more reliable than when it has been forged and welded into different and more complex forms.

The high temperature required in welding, demande experience and judgment in determining the proper time to "strike," that is, when the metal is hot enough to adhere firmly, but rot overheated to burning. Moreover, though the hammering required to bring

the parts together and reduce them to proper form and size, may prevent crystalization immediately at the welded point, still on either side are portions which may have been heated so as to change the arrangement of particles, and not subjected to sufficient hammering to counteract the deteriorating tendency. Hence, a break is more liable to take place a little on one side, than immediately through the welded part.

To obviate this liability, the parts to be welded should be enlarged by upsetting several inches from the end, so as to admit of re-drawing under the hammer a little beyond where the intense heat has reached.

But theory aside for the moment, although the avoidance of welding in work to be exposed to great stress is desirable, it is nevertheless a fact established by large experience, that welded parts will bear as great a strain as takes place in well proportioned bridge work, with as much certainty as ever has been realized in any department of the means of locomotion.

Danger lurks everywhere at all times. In railroad travel, boilers burst, rails break, wheels and axles break, etc., etc., but the failure of a weld in bridge work is rare indeed, and very few authenticated cases can be referred to.

I would, however, prefer a weld in the straight part rather than in the end of a link, unless made with an excess of section around the bend. Whether a bend around a pin of 1 or 2 times the diameter of the link iron is more liable to break than the straight sides of the link, I can refer to no reliable authority to deter mine. The longitudinal strain is no greater in the bended, than in the straight parts, if well fitted to the pin. But of course, it can not be expected to have a fit so close as to ensure a firm pressure quite round the