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C H A P. XXI. 

ON BRIDGES. 

	Thoughts on aqueducts, and their construction of iron, bear to near relation to bridges, that the ideas naturally tend to that subject, and hence I am led to offer some drawings on their formation of iron and wood.

	In this country the attention of engineers, of late years, have been much engaged in bridges of iron, which bridges of iron, which bridges are progressively expanding as experience produces courage; nor should I be surprised, if genius in time gave the mechanic rainbow of one thousand feet wide and rapid rivers.

	In such countries as Russia and America, the extensive arch seems to be a consideration of the first importance: in crossing their rivers, as the rivers, or even rivulets, in time of rain suddenly swell to a great height, and in the Spring, on breaking up of the ice, the immense quantities which is borne down with a rapid stream would, if interrupted by small arches and piers, collect to such a weight as ultimately to bear away the whole; it is therefore necessary that one arch should be extended as far as possible, in such situations, and so high as to suffer every thing to pass through; or the inhabitants must, without some other expedient, submit their passage to the casualties of the weather.

	The most extensive span of wooden bridges, as far as I am acquainted with the subject, are those of Schaffhaufen and Wettingen, in Switzerland: the first, constructed over the Rhine, is formed in two spans, one of one hundred and seventy-two feet, the other one hundred and ninety-three, amounting to three hundred and sixty-five, supported by one pier, relative to which there has been numerous arguments.

	The pier being the remains of an old bridge, and the artist having expressed his desire to cross the river in one span, or arch; but being over-ruled by the magistrates, who ordered him to give it a bearing on the pier, it is said he seemingly complied with their injunctions, but so contrived that no part should actually touch the pier; yet the pier is not in a line with the buttresses, but out of the rectilinear direction eight feet, forming an obtuse angle; and this circumstance is sufficient to convince me, that it must rest on the pier; therefore the greatest arch cannot be considered more than one hundred and ninety-three feet; yet certainly a considerable stretch of genius, and a strong instance of the curious fabric in which the frequently resides, the artist, Ubrick Grubenman, being a common carpenter, without the least knowledge of the principles of mechanics.

	In a drawing which I have seen, the leading beam, composed of two pieces laid on each other, rises in aspiring of about twenty feet over the pier, similar to the principles of a roof, and braced by perpendiculars and diagonals, in order that it may preserve its position, so that in some degree it operates like an arch, although in appearance the framing resembles a right line, the whole being roofed; a man on foot crossing this bridge will feel the whole fabric tremble, yet it is sufficient to support wagons heavily loaded, and bears every hardship usual to bridges.

	The bridge of Wittengen, over the Limmat, is a span of two hundred and forty feet, raised about twenty feet from the water, and may be said to hang between two bows, the system by which it is supported being a strong bow or arch composed of eight timbers bolted on each other to create breadth, and back up against the weight, one of the bows being on each side, forming a spring of about twenty-five feet; the horse is suspended between the two near the center of the bend, this is also roofed, and by the mode of combining, has more simplicity and true mechanism than that of Schaffhausen, although constructed by the same self-taught artist.