The shafts were sunk as pneumatic caissons to a depth of 78 ft. below mean high water.
"Transactions of the American Society of Civil Engineers, vol. LXVIII, Sept. 1910" by James H. Brace, Francis Mason and S. H. Woodard
Work was begun in May 1881, when attempts were made to sink an iron caisson under pneumatic pressure.
"Encyclopaedia Britannica, 11th Edition, Volume 16, Slice 6" by Various
Great Bridges, Pneumatic Caissons, Tunnels.
"The Progress of Invention in the Nineteenth Century." by Edward W. Byrn