BOILER PLATE TUBULAR BRIDGE. [Copy of a letter from the inventor.] Reading, May 1,1849. Dear Sir: -- Inclosed I send you the drawings of the three bridges I constructed on the Baltimore and Susquehanna Railroad while engaged as Superintendent of Machinery and Road. The one marked A was built at the Bolton dapet in Hi winter of 1645 and '7 and was put in its place in April, 1847. This bridge is made of puddled boiler-iron 1/2 inch in thickness. The sheets,standing ver-ticle, are 38 inches wide and 6 feet high, and riveted together with 5-8 rivets, two and a half inches from centre to centre of rivets. You will observe by reference to the drawing, that each truss-frame is composed of two thicknesses of iron, 12 inches distant from each other, and connected together by 5-16 iron bolts, passing through round cast iron sockets at intervals of 12 inches; which arrangement, together ?;ith the lateral bracing between the two trusses, which is composed of I round iron,set diagonally and bound together at the crossing by two cast-iron plates about 4 inches diameter, the sides next to the bracing being cut in such a manner, that when the two 5-8 bolts that pass through them were screwed uc, it held them firmly together. There is also a bolt passing throu-gh bottle truss-frames and through the heels of the lateral bracing, at right angles with the bridge, which secured the heels of the lateral braces, and by means of a socket in the centre made a lateral tie to the bridge, giving the bridge its lateral stability. The lower chords ware of hammered iron, there being some difficulty at that time to get rolled iron of the pr;cper size, and are in one entire piece, being welded together from bars 12 feet long. There are eight of them 5 >; I inches, one on either side of each piece of bdiler iron, and fastened to it with ?- inch iron rivets 6 inches distant from bottom, two on each truss near the top, the timber for the rail making up the deficiency for compression, and answering the purpose of chords. This bridge was built at the time Messrs. Stephenson and Brunei1 were making their experiments with cylindrical tubes preparatory to constructing the Menai bridge; the cylindrical tubes failing, they adopted this plan of brSdge. The entire weight of the bridge is 14 gross tons, and cost $2,200; but as the same kind of iron of which the bridge is composed car. be had for at least 15 per cent, lass now, than it cost at that time, it would be but fair to estimate the cost of the bridge at $1, 870, without any reference to the labor that is misapplied in all new structures of the kind, making the cost of a bridge 5c feet long $34 per foot. Anob I have no doubt, where there would be a large quantity of iron required for such turcoses. th*t it. cnnld be had at such prices as to bring down the cost of bridges of 55 feet length to $30 per foot. each other. There are but four top chords, and of Very respectfully yours, JAMES MILLROLLAND. Copy of old scrap from unknown newspaper of probiably May 1847. IRON BRIDGE A new patent Iron Bridge has recently been constructed ever Carroll's run, 18 mi Is 3 from Baltimore, by the Baltimore and Susquehanna Railroad Company. The bridge in the invention of Mr. Jas. Millholland, for which he has a patent; he has named it the "Howard Bridge". The following description of it we find in the American: "The bridge is "4 feet long, and is composed of two truss-frames,both of which are six feet deep. Each truss is made by placing sheets of iron of an inch thick, united tegc-ther at their edges, 12 inches apart, and are fastened to each other by stay bolts, which pass through tubes of cast iron placed between the two sides, at intervals of 12 inches, which secure the two sides composing the truss, firmly together. On each side of the lower edge of the sides of th truss are riveted straps of iron, 4 inches wide, and ?ths of an inch thick; these straps form the lower cords of the bridge. On the top there are but two cords, which are secured to the outside of the truss, near the upper edge. In the space between the sheets or sides, the track sjSrings are placed, and secured by bolts passing through that portion of the sides abeve the top cord;, and through the track strings. On the top of the track strings the rails are laid. The two truss frames ara secured to each other by bolts and diagonal braces of round iron -ths of an inch in diameter. The entire weight of the bridge is 2% CCC lbs., and was conveyed from the Eolton Depot of the Company tec its present place entire, and was placed upon the abu t m ents i n abo ut o ne hour. "The entire cost of the bridge is $2,200, which includes all the fixtures for making it. Another of the same dimentions could b4 built now for-.$15.-00. and if it had been built as was originally intended, that is, -4 feet deep, it uould not have exceeded $1, 000."