THE 





ANNUAL OF SCIENTIFIC DISCOVERY. 



MECHANICS AND USEFUL ARTS. 



PROPOSED GREAT IRON BRIDGE/ 



A GIGANTIC iron girder bridge, surpassing in dimensions the great 

 Britannia Bridge over the Menai Straits, is proposed to be construct- 

 ed across the Firth of Forth, about 17 miles from Edinburgh, 

 Scotland. Its length will be o,887 yards, or more than two miles, 

 with four spans of 500 feet each, over the navigable channel. 



The clear height of the bridge in the channel will be 195 ft. at 

 high water of spring tides, thus giving ample height beneath for the 

 tallest vessels. The frame of the girders over the widest spans will 

 be about 70 feet, giving a total visible elevation of 195 ft. for the dis- 

 tance of nearly half a mile, the height diminishing at the ends of the 

 bridge as the spans become reduced in width. Taking the sub- 

 merged part of the work, the height will be 25 ft. of foundation be- 

 low the silt bottom, 50 ft. of depth of water at ebb spring tides, and 

 18 ft. of fluctuation of tides, making in all 285 ft. in height of work 

 to be executed. The middle piers will be of stone to the height of 

 ten feet above high water at spring tides, and the rest of the struc- 

 ture will be of malleable iron. The dimensions of the Britannia 

 Bridge may be stated by way of contrast : Span, 460 ft. ; height, 

 104 ft, ; depth of tube, 30 ft. The cost of the bridge is estimated at 

 between 500,000 and 600,000. 



How a Chain Suspension Bridge is constructed. Most people un- 

 derstand how a stone bridge is erected, but the way in which the 

 chains of a suspension bridge are got across appears to puzzle many. 

 It is needless to say that the very modern invention of a suspension 

 bridge is only a bridge formed of powerful chains, buried in the 

 ground at their ends, and then built link by link over high towers, 

 the chains being of suiliricnt strength to support the road and foot- 



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