cylindrical (or conic segmental) brace b, running from each truck frame obliquely downward to an abutting block c, which is common to the two trusses, with chords or ties d, from truck to truck for each pair.

The truck wheels are from 20 to 24 inches in diameter, with 5 to 6 inches width of rim, and with short axles or shafts, 3 to 4 inches in diameter, according to dimensions of bridge. The axles run in journal boxes fitted to the truck-frame so as to bring the axles in the direction of radii to the circular track t, upon which the trucks are to run.

The truck frame consists of two cast iron side plates (of which g and h present an outside and an inside view), of an I formed cross section, and contour as seen at g. These plates upon the insides, have projecting portions as shown by the dark surface of diagram h, meeting from opposite plates, in the centre of the frame at a common surface of contact, and forming continuous tubes or sockets through the frame, which serve as media through which the ties d, act upon the cylindrical braces b, thus forming a rigid truss, which should be so proportioned as to be able to support (upon the two trusses), the whole weight of superstructure, throwing it upon the centre block c.

The chord ties d, of the two trusses, crossing one another upon the same level, are kept from mutual interference by cutting out the middle portion of one set, and replacing the removed part with two pieces to each tie bar, one passing above and the other below the single continuous rods of the other set, as shown at f

The block e has a cylindrical cavity in the under side, 10 to 12 inches in diameter, and about 7 inches deep, into which is fitted (loosely) a solid cylinder entering about 4 inches into the cavity, and leaving a space of

structure to be raised essentially free from bearing upsome 3 inches in thickness above, to be occupied by the nuts of a number of set screws s, intended to force down said internal cylinder upon the bed plate i, and thus relieve the truck wheels from nearly all the weight of superstructure.

The bed plate i, has a socket or step of an inch deep, or thereabouts, with a hardened steel plate in the bottom, to receive the lower part of the cylinder bearing upon the plate i, where the diameter of cylinder and socket should be graduated to the proportion most favorable for reducing the amount of friction. A diameter of 6 to 8 inches is thought to be suitable for draws of 60 to 100 feet opening, while the part of the pivot block within the block c should have a diameter of 10 or 12 inches, in order to afford sufficient surface for the set screws s to act upon.

The bed plate i, should have a rim about the step to retain oil, and the surfaces above and below the steel plate should have radial grooves to allow the penetration of oil; and these (grooves) should be so situated as to admit of their being probed, to prevent their getting clogged.

The pivot block should have guides to prevent its turning in the cavity of the block c; otherwise it might stick in the step, and the set screws slide upon its upper surface; which has been the case in some instances.

A groove should be formed in the under side of the block c, near the edge, to keep the water from the pivot; and the screws s, should be kept secluded from water by a tin, or galvanized iron cap shutting over a rim or ring cast upon the block c, outside of the screw holes. Sufficient vertical movement (1 or 2 inches), should be allowed to the pivot cylinder, to enable the elasticity of the braces and ties, b and d, to be taken up, and the

on the wheels e, as the bridge will move much more easily with the bearing upon the centre pivot, than up. on the truck wheels.

The king posts should be placed over the centres of trucks, or, when this can not be done, they should bear upon transverse beams which bear upon centres of trucks. In all cases, the bearing upon trucks should be through the medium of bolster plates so formed upon the under side as to touch the truck frame only upon a space an inch wide or less, square across the centre, as indicated by the parallel lines across the trucks in the large diagram of the Figure 74. Were the pressure applied in a line diagonally across the truck, it would act unequally upon the journals, and produce a tortion strain upon the truck frame, which the latter might not be able to bear.

Particular care should be taken to provide convenient means for keeping the working parts thoroughly oiled.

The superstructure being properly adjusted and balanced upon this turn-table, and the set screws s, forced down until all the truck wheels can be easily made to slide upon the rail by the use of a light crow bar, the structure will turn upon its centre pivot, steadied by contact of truck wheels upon the rail, with the least practicable resistance, and, during the transit of moving loads, the wheels, being in contact with the rail, are in readiness to sustain the additional weight without increase of pressure upon the pivot, or mcreased strain upon the diagonal trusses.

The modes and means for the application of power in working this table, as well as the preceding one, have already been described, [CLXXIX], and the description need not be repeated. They need no illustration by diagram, and are not shown in the drawings.

The plan, Fig. 74, requires the middle portion of the supporting pier to be depressed 1 to 2 feet, as shown at p, where a vertical section of the upper part of the pier is represented; and, under the bed plate i, should be a large and firmly bedded stone capable of sustaining the whole weight of superstructure.

This plan appears to answer all the requisites of a draw-bridge turn-table by the most direct and economical means.

LIFT DRAW BRIDGES.

CLXXXI. Under this designation may be included all movable bridges which are withdrawn from position by being raised, instead of moved horizontally out of place.

Lift bridges, though not much in use at the present day, have been constructed to be raised bodily, being counterpoised by weights acting over pulleys or sheaves; a plan scarcely feasible upon waters navigated by mast vessels, or steamers with high smoke stacks; as must be obvious on a moment's reflection.

The more common device for lift draws, is, to raise the platform from a horizontal to a vertical position, by lifting one end, while the other turns upon a hinge joint; the operation being like the raising of a trapdoor.

This plan is feasible over narrow channels, where vessels may be slowly warped through. But the process requires so much time as to seriously impede the land traffic. A bridge may be so balanced as to turn upon a horizontal axis about as easily as a swing bridge turns upon a vertical one. But the means available

for applying the counterpoise are far less convenient, being usually the action of weights over sheaves, and, the resistance constantly diminishing as the bridge rises, it requires a complicated arrangement to graduate the action of the counterpoise to an equality with the resistance at all stages of the movement. Still, the thing may be practicable, were the object of sufficient utility to warrant the undertaking. For instance, the counterpoise may be permanently attached to the draw in such position as to bring the common centre of gravity in the line of the axis of motion; when the only resistance would be the friction of the journals at the hinge joint. Again, a counterpoise acting upon a windlass might raise the draw by chains winding upon a fusee, with radius increasing as resistance diminishes. Or, weight might be mounted upon wheels, and run down upon a curved incline, so adjusted as to diminish its action to an equality with the resistance at the dif ferent stages.

But none of these devices are suitable for effecting more than very small openings, and are not likely to be often adopted. They will therefore be passed by with a mere allusion.

Lift bridges have also been constructed to open in the middle and lift both ways. By this means wider openings may be effected.

But, as the middle portion of the bridge and passing loads must be sustained by the lifting chains, this plan is not well adapted to any but light traffic. Such a structure over the Albany Basin broke down many years ago with fatal results. Perhaps, however, the catastrophe resulted rather from the imperfect condi tion or faulty construction of the bridge, than from in herent defects of the general plan.