Instead, therefore, of occupying the reader's time with a mathematical discussion, from which, at pre sent, little benefit can be derived, we conceived it bet ter to offer a few practical remarks on the methods by which the chief difficulties of such a case may be obviated.
With this view we will venture to recommend, that whatever the position or the bridge may be with re spect to the stream, the lengthway of the piers and abut ments should coincide with the direction of the current as nearly as may be. A considerable deviation is per fectly practicable. We have formerly shewn, that the position of the joints of the arch may be varied, perhaps 30° from that of equilibration, before any slide is thereby produced among the archstones : we surely have the same liberty here, and this even when the coursing joints are horizontal.
This advantage of friction may be even improved by dowelling or other means, if thought beneficial. But if the coursing-joints, instead of being made horizontal, be formed at right-angles to the side of the bridge, we have the very same advantages that would be obtained were the bridge perfectly direct.
Did the subject admit of it, we would here give the theory of those arches which are not straight on the ground-plan, as the arched top of a bow-window, and the like, sonic of which afford pretty examples of the appli cation of the principles of equilibration, and all of which, by means of the gravity, friction, or cohesion of their parts, admit to a certain extent ot being steadily and solidly constructed. But we shall, at another oppor tunity, take up this subject, and, in the meantime, Ave hasten to other matters more intimately connected with our present inquiries.
The centres or framings of carpentry, on which the arches are built, are, in many cases, objects of great anxiety to the builder : they form one of the most beauti ful applications of the science of CARPENTRY. But as the principles upon which the more difficult kinds are designed will not be clearly understood, without enter ing- into more detail than is consistent with our present subject, we shall only offer in this article some of the most approved designs, and refer the reader for their explanation and discussion of their comparative merits until we come to treat of CARPENTRY.
To the same place we shall also refer the subject of wooden and iron bridges, which intimately depend on the same science ; their statical equilibrium admitting of the introduction of a principle essentially different from those employed in erections of stone.
The side-walls and wing-walls of bridges constitute a part not the least important of our subject. They have, especially the latter, to resist the pressure of a mass of earth behind them, forming the roadway up to the haun ches of the bridge. Little has been written upon this subject in our language, and that little has been of no practical benefit. Muller and some others give us a few theorems respecting the thickness of the revetments in fortifications. Some of our elementary writers inquire a little into the same subject, as one case of the applica tion of the doctrine of the centre of gravity. Their results, in almost every case, give a much greater thick ness for walls of this kind, than is ever found necessary in practice. There can be no doubt that some principle must be overlooked in these investigations, which has a