In no previous period of the world's history could this Bridge have been built. Within the last hundred years the greater part of the knowledge necessary for its erection has been gained. Chemistry was not born until 1776, the year when political economy was ushered into the world by Adam Smith, and the Declaration of Independence was proclaimed by the Continental Congress, to be maintained at the point of the sword by George Washington. In the same year Watt produced his successful steam engine, and a century has not elapsed since the first specimen of his skill was erected on this continent. The law of gravitation was indeed known a hundred years ago, but the intricate laws of force, which now control the domain of industry, had not been developed by the study of physical science, and their practical applications have only been effectually accomplished within our own day, and, indeed, some of the most important of them during the building of the Bridge. For use in the caissons, the perfecting of the electric light came too late, though, happily, in season for the illumination of the finished work. This construction has not only employed every abstract conclusion and formula of mathematics, whether derived from the study of the earth or the heavens, but the whole structure may be said to rest upon a mathematical foundation. The great discoveries of chemistry, showing the composition of water, the nature of gases, the properties of metals; the laws and processes of physics, from the strains and pressures of mighty masses to the delicate vibrations of molecules, are all recorded here. Every department of human industry is represented, from the quarrying and the cutting of the stones, the mining and smelting of the ores, the conversion of iron into steel by the pneumatic process, to the final shaping of the masses of metal into useful forms, and its reduction into wire, so as to develop in the highest degree the tensile strength which fits it for the work of suspension. Every tool which the ingenuity of man has invented has somewhere, in some special detail, contributed its share in the accomplishment of the final result. "Ah! what a wondrous thing it is To note how many wheels of toil One word, one thought can set in motion." But without the most recent discoveries of science, which have enabled steel to be substituted for iron--applications made since the original plans of the Bridge were devised--we should have had a structure fit, indeed, for use, but of such moderate capacity that we could not have justified the claim which we are now able to make, that the cities of New York and Brooklyn have constructed, and to-day rejoice in the possession of, the crowning glory of an age memorable for great industrial achievements.