1018

THEORY AND PRACTICE OF ENGINEERING.

Book II.

For the removal of the earth necessary to construct the foundation of the bridges builtby Perronet, small carts were used having a triangular section in the line of their draught;each was mounted on a pair of wheels, and two attached together would be drawn by onehorse : a part of the load rested below the axle, which passed through the middle of that

PEKRO.NET’S CARTS.

Fig. 1540.

part of the cart which retained it; and as the whole was nicely balanced and swungupon its centre of gravity, when it was to be discharged, a very small power only wasrequired to turn it over and empty it of its contents.

The shafts and frame were of timber, and the axle of iron; the wheels were ofconsiderable diameter, and placed wide apart, to prevent their being easily upset; the tem-porary or provisional roads being often far from level, this precaution was peculiarly

Fig. 1541. PLAN OF CARTS.

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necessary. In the rear of each cart, and in the middle of the cross tie or brace, was aniron ring and pin, which allowed of another cart, having a pole instead of shafts, to beattached to it.

The transverse sections and end view of these convenient carts show the manner theywere suspended, and the diagram between them exhibits the difficulty of overturning afigure so hung.

Fig. 1542.

SECTIONS AND ENDS OP CARTS.

Two carts contained together about a cube yard, and were found extremely serviceable;they could be easily disconnected in case of accident, and attached with facility, before orafter they were loaded.

The body of the waggons employed on the railroads for the conveyance of coals is madelike the frustum of a pyramid, although in some instances the square form is preferred:about the middle of the last century iron wheels -were introduced for the carriages em-ployed at the coal mines near Newcastle, and they are now usually of cast-iron; the rimis made cylindrical, with a projection on one side, called the flanch, which keeps it uponthe rail; the axles are of wrought-iron, and fixed in the centres of the wheels.

Waggons for conveying coals usually' discharge their contents at the bottom, on whichaccount they are preferred of this shape; the framework consists of two side soles fastenedtogether by four cross shetbs well bolted. The upright sheths and top framing are ofiron, and the ends and sides are dead with deals bolted to the upright sheths ; sheet-ironis sometimes preferred to the deals, when it is riveted on to iron uprights.