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THE ORIGINAL PREFACE.

TRACT 1 .

tions of the various parts of a bridge, are strictly demon-strated, and clearly illustrated by various examples. It isdivided into five sections: the 1st treats on the projects ofbridges, containing a regular detail of the various circum-stances and considerations that are cognizable in such pro-jects. The 2d treats on arches, demonstrating their variousproperties, with the relations between their intrados and ex-trados, and clearly distinguishing the most preferable curvesto be used in a bridge; the first two or three propositionsbeing instituted after the manner of two or three done byMr. Emerson in his Fluxions and Mechanics. The 3d sec-tion treats on the piers, demonstrating their thickness ne-cessary for supporting any kind of an arch, springing at anyheight, both when part of the pier is supposed to be im-mersed in water, and when otherwise. The 4th demonstratesthe force of the water against the end or face of the pier,considered as of different forms; with the best form for di-viding the stream, &c. and to it is added a table, showingthe several heights of the fall of the water under the arches,arising from its velocity and the obstruction of the piers; asit was composed by Tho. Wright, Esq. of Auckland , in thecountv of Durham , who informs me it is part of a work onwhich he has spent much time, and with which lie intends tofavour the public. And the 5th and last section contains a'Dictionary of the most material terms relating to the sub-ject: in which many practical observations and directionsare given, which could not be so regularly nor properly in-troduced into the former sections. The whole, it is pre-sumed, containing full directions for constituting and adapt-ing to one another, the several essential parts of a bridge, soas to make it the strongest, and the most convenient, bothfor the passage over and under it, which the situation andother circumstances will admit: not indeed for the actualmethods of disposing the stones, making of mortar, or theexternal ornaments, &c. those things are not here attempted,but are left to the discretion of the practical architect, asbeing no part of the plan of this undertaking; and for the