a crushing strength of 50 tons per square foot. Above low water the cylindrical piers which are 49 feet in diameter at the top 55 feet at the bottom and 36 feet high consist of the strongest masonry the hearting being flat bedded Arbroath stone and the facing Aber deen granite. In each cylindrical pier there are 48 steel bolts1% inches in diameter and 24 feet longto hold down the bed plate and superstructure of the main spans. One of the Fife piers was built by aid of a tim ber and clay cofferdam and one by means of a half tide dam. At Inch Garvie much of the work of the shallow piers had to
a crushing strength of 50 tons per square foot. Above low water the cylindrical piers which are 49 feet in diameter at the top 55 feet at the bottom and 36 feet high consist of the strongest masonry the hearting being flat bedded Arbroath stone and the facing Aber deen granite. In each cylindrical pier there are 48 steel bolts1% inches in diameter and 24 feet longto hold down the bed plate and superstructure of the main spans. One of the Fife piers was built by aid of a tim ber and clay cofferdam and one by means of a half tide dam. At Inch Garvie much of the work of the shallow piers had to be done at low tide under great difficulties. The Queensferry Pier consists of a group of four cyl indrical caissons 70 feet in diameter at the bottom edge. Owing to the special conditions of the site the work differs in some respects from ordinary pneumatic caissons. The bed of the Forth at the Queensferry Pier is of very soft mud for a depth of from 20 feet to 35 feet when the bowlder clay is reached the surface of both the mud and the clay falling sharply toward the 200 feet deep channel between Queensferry and Inch Garvie. The caissons had to be floated out and sunk about one-third of a mile from shore in an expos ed seaway. To facilitate operations the caissons have double skins 7 feet 6 inches apart and vertical bulk heads between the skins. By filling the space between the skins with concrete to varying heights the irregu larity in the level of the bottom and the hardness of the mud could be to some extent compensated for as the weight brought upon the cutting edge of the caisson could be regulated as desired. Iron being cheap a liberal use was made of it in conjunction with concrete where masonry or brickwork might have been em ployed. Strong lattice girders and cross girders stiffen to the required extent the roof of the working chamber. These girders are subject to a heavy bending stress up ward and downward owing to the tide the range of which is about 20 feet. Thus i
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