SECT. 5 .

A DICTIONARY OP THE TERMS.

113

driven about the pier as a border of defence, or to supportthe centres on ; and in this case, when the centring is re-moved, they must either be drawn up, or sawed off very lowunder water ; but it is perhaps better to saw them off, andleave them sticking in the bottom, lest the drawing of themout should loosen the ground about the foundation of thepier. Those to build on, are either such as are cut off by thebottom of the water, or rather a few feet within the bed ofthe river; or else such as are cut off at low-water mark, andthen they are called stilts. Those to form borders of defence,are rows driven in close by the frame of a foundation, to keepit firm ; or else they are to form a case or jettee about thestilts, to keep within it the stones that are thrown in to fill itup ; in this case, the piles are grooved, driven at a small dis-tance from each other, and plank piles let into the groovesbetween them, and driven down also, till the whole space issurrounded. Besides using this for stilts, it is also sometimesnecessary to surround a stone pier with a sterling or jettee,and fill it up with stones to secure an injured pier from beingstill more damaged, and the whole bridge ruined. The pilesto support the centres may also serve as a border of piling tosecure the foundation, cutting them off low enough after thecentre is removed.

Pile Driver, is an engine for driving down the piles. Itconsists of a large ram or square block of iron, sliding per-pendicularly down between two guide posts; which beingdrawn up to the top of them, and there let fall from a greatheight, it comes down on the top of the pile with a violentblow. It is worked either by men or horses, and either withor without wheel work. That which was used at the build-ing of Westminster-bridge, is perhaps one of the best kind.

Pitch, of an arch, is the perpendicular height from thespring, or impost, to the keystone.

Plan, of any part, as of the foundations, or piers, or su-perstructure, is the orthographic projection of it on a planeparallel to the horizon.

Push, of an arch, the same as drift, shoot, or thrust.

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