HISTORY AND PROGRESS

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the top, or bearing surface ; the rails, therefore,either rest upon the flat base of the chair, orupon the pins. When the pins do not fill theholes, the rails will of course rest upon thechair; but, if the pins are driven tightlythrough the holes of the rails, they will ne-cessarily be supported by the pins ; and, ineither case, the parallelism of the surface of therails will depend upon the parallelism of thebase of the chair with the line of the road.

If the surface of the ground on which thestone rests be not of the same degree of firm-ness throughout, or the chair be not placedprecisely in the centre of, and parallel with,the bearing section of the stone, the weightof the carriages passing along the rail willdisplace the stones, by moving them from theirparallelism with the line of the road, andthrowing them down on one side into the posi-tion represented at c c, Fig. IX. Plate I. Thisnecessarily depresses one side of the base ofthe chair, and also one of the pins, below theother, and consequently depresses the end ofthat rail fastened to it below the line of theother, as shewn at ri d . And this derangementof the rails will take place whenever the lineof the base of the stone does not correspondwith the line of the road ; and will be in pro-portion to the angle the one forms w ith the other