In these Figures every part of the construction is so particularly delineated, as to render its office evident by inspection. The braces proceeding from each abut ment, are continued to the beam which passes along the top of the uprights, and the lowest ol these general bra ces are actually united under that beam, thereby forming a continued arch between the abutments, the chord line of which is 364 feet, and the versed sine about 30 feet. These braces are kept in a straight direction by the up rights, which are placed 17 feet 5 inches apart. If this bridge had been formed in a straight line between the abutments, we can see no reason NI by this form of con struction should not have supported a roadway of about 18 feet in breadth, as well as a slight roof ; because, in that case, all the weight arising from the braces which proceed from the middle pier would have been saved, and the roof might have been made much simpler and lighter ; but the general direction being 8 feet out of a straight line, and being loaded with an unnecessary heavy roof, it was certainly advisable to make use of the braces from the middle pier, and thereby composing two distinct arches.
Although the principles, and even the form of con structing this bridge, might have been drawn from Gau tier's publication, or even Palladio's designs for wooden bridges, yet from the account of Ulric Grubenman, being an illiterate man, there is reason to think it was from his own inventive genius that the whole design originated. There is not only a great boldness in the principal mem bers, but also a wonderful attention to all the immix of the edifice; and from even the roofing being overdone in aiming at excellence and security, it is evident this was a first attempt, and that there was an anxiety to avoid the possibility of failure, in what he conceived, and what, as lar as regards him, was really a totally new project.
We arc informed that John Grubcnman constructed a bridge upon the same principles, of 24u feet span, over the R iue, near Rienenau ; also that the two brothers crect‘d one 200 feet span over the river Linimat, near Baden. And that the last work of Ulric was a bridge
of 230 feet span at Wittingen. In this last, the form of construction was varied : instead of placing the braces diverging from each other, st.ven beams were built close upon each other, forming a catenarian arch between the abutments, of which the rise was 25 feet. These beams were of oak, in lengths of 12 or 14 fcci, breaking joint in the manner of masonry. They were not fastened by pins, bolts, or scarfings; but were kept together by iron straps, placed five feet distant from each other, and fastened by bolts and keys. The roadway intersects them about the middle of their rise.
Over the river Portsmouth, in North America, a Mr Bludget has constructed a wooden bridge 250 feet span, nearly in tlfe same form as the last mentioned of Gru henman ; that is to say, each truss or arch consists of three rows of beams placed parallel with, but at sonic distance from, each other, and each beam consists of two halves, connected by dovetailed keys passing through them horizontally ; and similar keys are also passed ver tically through all the three beams. This has a more elegant appearance, than where the beams are laid close together ; but we doubt if the frame is equally firm.
We have now given a succinct account of the different modes in which wooden bridges have been constructed : from this, the practical mechanic may judge of their comparative merits, and the propriety of their applica tions in the various situations occurring in practice. Their principles will be discussed under the head of CARPENTRY, as connected with roofing, centres, domes, &c. and we must therefore refer the reader to that article for further information upon this useful and generally interesting subject.