Their spans will be known by dividing the whole breadth of the river into a convenient number of arches and piers, allowing at least the necessary thickness of This and the disposition of it above, so as to ren∣der the passage over it also convenient, make up its utility.-Having fixed the heights of the arches, their spans are still necessary for de∣termining their figure. They ought to be made so high, as that they may easily transmit the water at its greatest height either from tides or floods and their height and figure ought also to be such as will easily allow of a convenient passage of the craft through them. The form of the arches will depend on their height and span and the height on that of the water, the navigation, and the adjacent banks.
With respect to the form strength, utility, and beauty ought to be regarded and united the chief part of which lies in the arches.
HAVING resolved on the place, the next con∣siderations are the form, the estimate of the ex∣pence, and the manner of execution. These are the chief and capital articles of con∣sideration, and which will branch themselves out into other dependent ones, and so lead to the required estimate of the whole. Of the stream, depend the manner of laying the foundations, and building the piers. The nature of the bed of the river is also of great concern, it having a great influence on the expence as upon it, and the depth and velocity However, in most cases, a wide part of the river ought rather to be chosen than a narrow one, especially if it is subject to great tides or floods for, the increased velocity of the stream in the narrow part, being again aug∣mented by the farther contraction of the breadth, by the piers of the bridge, will both incommode the navigation through the arches, and under∣mine the piers and endanger the whole bridge. The breadth of the river, the navigation upon it, and the quantity of water to be passed, or the velocity and depth of the stream, from also considerations of great moment as they deter∣mine the bridge to be higher or lower, longer or shorter. The banks or declivities towards the river are also of particular concern, as they affect the conveniency of the passage to and from the bridge, or determine the height of it, upon which in a great measure depends the expence. If possible, the bridge should be placed where there can conveniently be opened and made passages or streets from the ends of it in every direction, and especially one as nearly in the direction of the bridge itself as possible, tending towards the body of the town, without narrows or crooked windings, and easily communicating with the chief streets, thoroughfares, &c.-And hereĮvery person, in judging of this, should divest himself of all partial regards or attachments what∣ever think and determine for the good of the whole only, and for posterity as well as the pre∣sent. In this estimation, a great number of particu∣lars must be included and nothing omitted that can be found to make a part of the consideration.Īmong these, the situation of the town or place for the convenience of which the bridge is chiefly to be made, will naturally produce a particular of the first consequence and a great many others ought to be sacrificed to it.
And the same being done for this and a third, and so on, the best situation of all will be ob∣tained.
And by doing the same for any other situations, all their neat values will be found, and of consequence the most preferable situation among them.-Or, in a competition between two place, if each one's advantage over the other be estimated or valued in every circum∣stance attending them, the sums of their advan∣tages will shew whether of them is the better. In agitating this most important question, e∣very circumstance, certain and probable, attend∣ing or likely to attend the bridge, should be separately, minutely, and impartially stated and examined and the advantage or disadvantage of it rated at a value proportioned to it: then the difference between the whole advantages andĭisadvantages, will be the neat value of that particular situation for which the calculation is made. WHEN a bridge is deemed necessary to be built over a river, the first consideration is the place of it or what particular situation will contain a maximum of the advantages over the disadvantages. Of the Projects of Bridges, with the Design Estimate, &c.
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