Suppose, now, the waters to rise even higher than the keystone, the weight of the keystone itself being di minished, the arch will be in the very same predica ment as if it were formed entirely of materials of a smaller specific gravity than before, and its chicf dan ger will arise from the transverse action of the stream tending to overset it.
This will be the case when the water, having free in gress through the materials, or through the gutters of the bridge, rises as fast in the interior of the building as without. But this is not always to be expected. The side walls, or parapet, may be so formed, as not to ad mit the water to enter, at least not with sufficient rapi dity. The arch itself, we are sure, will not, for it is all laid in mortar. Now, in the event of the arch being formed with open work in the haunches, it will not, we think, be going too far to say, that there may be a point to which, if the waters arrive, the whole weight of the arch may be balanced by the hydrostatic pressure upon the intrados ; and in that case, it would be shoved off in one mass by the pressure of the stream.
This is by no means even an improbable supposition, for the key-stone itself will begin to move whenever the waters rise one and a half times its thickness above the solid matter at the crown ; and it will readily be granted, that every other section is pressing strongly upwards by that time. It may, indeed, be alleged, that the pressure
of dead weight over them would keep them down long after that, and this we do not deny ; but the derange ment which it is likely will have taken place among the lower stones, by such a pressure acting from the points of the wedges, will, in all probability, be such as to ren der the destruction of the arch inevitable.
For example, take a stone of a foot square, and 4 feet deep in the soffit, near the springing of an arch of 40 feet rise ; suppose the arch full, this stone is pressed back with the weight of 40 cubic feet of water ; that is, a force of four times its own weight, and as a similar force, though gradually lessening, acts upon every other stone to the crown of the arch ; it is we think, very ob vious, that their united effect is likely to be of much more consequence than the thrust of the archstones.
But we may find another opportunity for rendering these motions somewhat more precise, by subjecting the forces to calculation, when we come to treat of Cur, VERTS, under INLAND .Vavigation, the chief case where such a process is likely to occur ; and which, from that circumstance, require some peculiar maxims of con struction.