ENGINEERING. 581 



BRIDGING THE FIRTH OF FORTH. 



A project which will present, if possible, even greater diffi- 

 culties than the construction of the Tay Bridge, elsewhere 

 referred to, is the bridging of the Firth of Forth, which, it is 

 said, has been decided upon, and so far advanced that plans 

 have been selected, and the immense capital that will be re- 

 quired for the work actually secured. The bridge, from 

 what has transpired, is to be a two -span suspension struct- 

 ure, the depth of the estuary making it impossible to lind 

 foundation for piers save on the Island of Inchgarvie. On 

 each side of the Firth will be located a composite pier, com- 

 posed of four sets of iron columns, resting on substantial base- 

 ments, and immense chains, securely attached some distance 

 inshore, will be carried over the tops of these piers and of 

 two smaller ones on the island above named. These chains 

 will support the two lattice girders, each 1600 feet in length. 

 The height of the spans will be 135 feet above high water. 

 The approaches on each side from the high ground to the wa- 

 ter's edge will be a series of spans carried on brick columns. 



A RAILWAY BRIDGE ACROSS THE TAY, AT DUNDEE, 



said to be one of the longest bridges in the world, and to 

 have been a work of extreme difficulty, was completed dur- 

 ing the past year. Its exact length is 10,612 feet. The num- 

 ber of spans is 85, of varying width. The chief difficulty 

 which the engineers encountered arose from the varying 

 character of the bed of the river. Near the shore the rocky 

 bed was easily reached, and the piers thereon were built 

 throughout of brick. Further out, however, the rock- bed 

 suddenly shelved downwards to a great depth, and was over- 

 laid w r ith clay and gravel. Here it was necessary to con- 

 struct, at great labor, large cylinders of masonry filled in 

 with concrete, as a foundation, on which, above high-water 

 mark, iron columns supporting the superstructure were erect- 

 ed. The structure at the centre of the stream is 130 feet 

 above high-water mark. 



THE RAILWAY BRIDGE 



over the Duoro River, which was completed during the past 

 year by the Northern Portugal Railway, is described as a 



