He died of lockjaw just sixteen days after receiving his injuries." For a time work on the bridge was paralyzed. As soon as possible the directors chose Colonel Roebling to succeed his father, and the great undertaking proceeded. The mechanical difficulties of the work were enormous. The history of the labors, by which one difficulty after another was overcome, is one of the most absorbing in the annals of engineering enterprise. Huge wooden caissons were sunken on the diving-bell principle to a depth sufficient to assure firm foundations for the piers, which were built over them. The Brooklyn caisson was launched on March 19, 1870; the New York caisson, in September, 1871. The greater difficulties existed on the New York side, where an area of quicksand made it problematical whether bed-rock could ever be reached. The foundation on the New York side was required to be begun at a depth of seventy-eight feet. On the Brooklyn side brick was used under the caisson. On the New York side the space remaining after the lowest point had been reached was filled with concrete. The most perplexing problem having been solved by the sinking of the foundations, the work advanced steadily. Difficulties with anchorages, materials, contracts, expenditures, and appropriations made the work necessarily slow, and there was a proportionate degree of public impatience. The distant possibility of a completed bridge was the permanent theme of newspaper jest and popular song. But the Brooklyn tower, containing 38,214 yards of masonry, and rising 278 feet above high water, was completed in the spring of 1875, and by the summer of 1876 the New York tower had also been finished. During this period the pressure on the various city ferries was demonstrating the necessity for some relief to the strain of travel between the two cities. During the year 1869 the Union Ferry Company carried 42,720,000 passengers; the Roosevelt, Grand, and James Slip ferries, 7,028,000 passengers; the Greenpoint, 1,622,250; and the Thirty-fourth Street, 2,250,550. The terms of the new lease of the Union Ferry Company included a provision that the fare between five and half-past seven o'clock, morning and evening, be one cent. It was a few months later that the Brooklyn City Railroad Company reduced its rate of fare to five cents. Mayor Kalbfleisch was reëlected Mayor. In his message of January 3, 1871, he places the population of the city in 1870 at 400,000; the taxes levied during the year at $8,000,000; the city debt at $36,000,000. The period was active in building operations. The foundations of the still unfinished Roman Catholic Cathedral were laid in 1868. The Twenty-eighth Regiment armory was completed in 1870. The Brooklyn Theatre was begun early in the following year, shortly before the finishing of the new wing of the Long Island College Hospital, and the laying of the corner-stone of the Church Charity foundation at Albany Avenue and Herkimer Street.