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THE PRINCIPLES OF BRIDGES.

TRACT I.

propositions, and numerous observations, botli practical andscientific. To the end is also added an Appendix, being theauthor’s report to the Committee of Parliament, on the pro-ject for a new cast-iron bridge, of one arch, over the river atLondon; and several other appropriate appendages.

SECTION r.

ON THE PROJECTS OF BRIDGES; WITH THE DESIGN,TIIE ESTIMATE, &C.

When a bridge is deemed necessary to be built over ariver, the first consideration is the place of it; or what par-ticular situation will contain a maximum of the advantagesover the disadvantages. In agitating this important ques-tion, every circumstance, certain and probable, attending orlikely to attend the bridge, should be separately, minutely,and impartially stated and examined; and the advantage ordisadvantage of it rated at a value proportioned to it; thenthe difference between the whole advantages and disadvan-tages, will be the net value of that particular situation forwhich the calculation is made. And by doing the same forother situations, all their net values will be found, and ofconsequence the most preferable situation among them.—Or, in a competition between two places, if each one’s ad-vantage over the other be estimated or valued in every cir-cumstance attending them, the sums of their advantages willshow which of them is the better. And the same being donefor this and a third, and so on, the best situation of all willbe obtained.

In this estimation, a great number of particulars must beincluded; nothing being omitted that can be found to makea part of the consideration. Among these, the situation ofthe town or place, for the convenience of which the bridge