16 ANNUAL OF SCIENTIFIC DISCOVERY. 



way afterwards to be hung from them. The chains, or rather joint- 

 ed iron bars, as they really are, are built together by joining side- 

 ways a series of flat iron bars, or parts of links, ten and eleven 

 bars, an odd and even number, alternately composing one link ; and 

 the bridge having one, two, or three chains of numerous links at 

 each side, according to the weight it is built to carry and the dis- 

 tance it has to cross. The first thing done is to get the anchorage 

 of the shore-ends of the chains at each side. This, in some of the 

 great suspension bridges, is done by taking down the links no less 

 than 75 ft. deep into the heart of the limestone rocks, and then build- 

 ing them in with solid masonry. The series of bars composing the 

 links are then embedded at each side, and from these moorings first 

 a thin rope is passed over the towers and across to the other side, 

 then a thick rope is conveyed along the thin, and lastly a wire rope 

 along the thick one. With two wire ropes thus got across, a cradle 

 is slung between them, and the workmen can then go on placing and 

 bolting together as they take a single bar or plank of iron of each 

 link in the chain. The wire ropes are strong enough to support 

 their weight till they are joined in the middle, and thenceforth they 

 support themselves. It is then simply a question of the cradle trav- 

 eling to and fro till all the bars of each link have been added on 

 every additional bar, making the whole work stronger. 



THE MT. CEXIS TUNNEL. 



At. a recent meeting of the Institution of Civil Engineers, London, 

 Mr. Sopwith, in a paper on the progress of this great work, said, 

 that, at the average rate of two metres per day, from June 30, 

 1864, six years and seven months would be required for the comple- 

 tion of the tunnel. An immense advantage had been gained in the 

 rate of execution by the use of the boring-machines of M. Sommeil- 

 ler's system, so that the above period would suffice for what would 

 otherwise have occupied twenty-six years and three months by hand 

 labor, at the rate of 1,665 feet per day at each end, the average pro- 

 gress before the use of the machinery. These machines did not 

 weigh more than 600 weight, and could pierce a hole about one 

 and a quarter inch diameter, and three feet deep, into a rock in 

 twenty minutes. It would occupy a couple of workmen two hours to 

 do so much. The machine consists of two parts : one, a cylinder for 

 propelling the borer against the rock ; the second, a rotary engine 

 for working the valve of the striking cylinder, turning the borer on 

 its axis at each successive stroke, and advancing or withdrawing the 

 cylinder, striking as occasion requires. It gives 250 blows per min- 

 ute. The effective pressure on the piston in striking was 216 

 pounds. Compressed air was used to drive the machinery and sup- 

 ply fresh air to the workmen. It was used at a pressure of five at- 

 mospheres above atmospheric pressure, and was conveyed to the 

 front of the advanced gallery by a pipe. Holes were bored in the 

 front by the machine, which was then withdrawn, and a gang of men 

 charged the holes with gunpowder and fired them ; another set of 

 men removed the debris. 



