MECHANICS AND USEFUL AKTS. 37 



feet and the rest of 242 feet, witli a headway of 60 feet. The 

 Britannia Bridge over the Menai Straits is 1,487 feet long with- 

 out the abutments, with two spans of 230 feet each, one of 458 

 feet 8 inches, and one of 459 feet ; and the Saltash Bridge 468 feet. 

 The Forth Bridge has a lengtli of 10,550 feet; and the Severn 

 Bridge nearly 12,000 feet. The bridge of the Hartford, Spring- 

 field and New Ilaven Railroad, over the Connecticut, at Ware- 

 house Point, replaces a wooden structure on stone jjiers, and was 

 built on the old piers with the addition of several new ones in tlie 

 same line, so that the present structure occupies the exact site of 

 the former one ; and during the seven months of its construction 

 no delay of trains was caused by the work. This is remarkable 

 when the magnitude of the work is considered. The bridge is 

 l,524i feet in length, composed of 624 tons of wrought-iron, 

 the iiooring only being of wood. In its construction, 175,000, 

 rivets were used. The cost was $264,784.63, and it is capable of 

 bearing a strain of two and a half tons to the foot. I'he iron- 

 work was made in England, by Fairbairu & Co. of Manchester, 

 and the London Engineering and Iron Ship-Building Company. 

 The plans and designs were by James Laurie, Civil Engineer, of 

 Hartford, Conn. — Scientific American. 



STEEL BRIDGES. 



At a late meeting of the Literary and Philosophical Society, 

 Mr. S. B. Worthington, C. E., stated that he had lately con- 

 structed a swing bridge for carrying a i-ailway over the Sankey 

 Canal, in which the girders were made of Bessemer steel plate. 

 The object of using steel instead of wrought-iron was to reduce the 

 weight of the girders ; these are four in number, about 56 feet 

 long, with bearings varying from 30 to 40 feet, and 2 feet deep. 

 They wei'e manufactured from steel tubes made by the Bolton 

 Steel and Iron Company ; and were tested with loads of a ton to 

 tlie foot, or more than double the weight which they could possi- 

 bly be called upon to bear. The deflection varied from one-half 

 to one inch, according to the length of the girder, and there was 

 no permanent set on removal of the testing load. The plates 

 used varied from one-quarter to seven-sixteenths of an inch in 

 thickness ; and the average tensile strength of a considerai^le 

 number of plates tested was upwards of 36 tons to a square inch. 

 The weight of the girders was about five-eighths of the weight 

 which they would have been if wrought-iron had been used. 



CONCRETE BLOCKS FOR BUILDING. 



An ingenious application of the well-known process of mould- 

 ing blocks of couereto for building purposes was patented some 

 time back. The inventor, a Mr. Tall, jDroposes to erect walls, 

 houses, and other structures, by literally casting them of concrete, 

 in the place they are intended to occupy. An ordinary concrete 

 foundation is first laid, and ui^on the foundation horizontal frames, 

 constructed of boards lined with zinc or other metal, are set up 

 on edge, so as to form a kind of trough for receiving the concrete. 

 4 



