One of the most primitive tokens of civilization is a bridge; and yet no artificial object is more picturesquely associated with its ultimate symbols: the fallen tree whereon the pioneer crosses a stream in the wilderness is not more significant of human isolation than the fragmentary arch in an ancient city of the vanished homo of thousands. Thus, by its necessity and its survival, a bridge suggests the first exigency and the last relic of civilized life. The old explorers of our Western Continent record the savage expedients whereby water-courses were passed,--coils of grape-vine carried between the teeth of an aboriginal swimmer and attached to the opposite bank, a floating log, or, in shallow streams, a series of stepping-stones; and the most popular historian of England, when delineating to the eye of fancy the hour of her capital's venerable decay, can find no more impressive illustration than to make a broken arch of London Bridge the observatory of the speculative reminiscent. The bridge is, accordingly, of all economical inventions, that which is most inevitable to humanity, signalizing the first steps of man amid the solitude of Nature, and accompanying his progress through every stage of civic life: its crude form makes the wanderer's heart beat in the lonely forest, as a sign of the vicinity or the track of his kind; and its massive remains excite the reverent curiosity of the archaeologist, who seeks among the ruins of Art for trophies of a by-gone race. Few indications of Roman supremacy are more striking than the unexpected sight of one of those bridges of solid and symmetrical masonry which the traveller in Italy encounters, when emerging from a mountain-pass or a squalid town upon the ancient highway. The permanent method herein apparent suggests an energetic and pervasive race whose constructive instinct was imperial; such an evidence of their pathway over water is as suggestive of national power as the evanescent trail of the savage is of his casual domain. In the bridge, as in no other structure, use combines with beauty by an instinctive law; and the stone arch, more or less elaborate in detail, is as essential now to the function and the grace of a bridge as when it was first thrown, invincible and harmonious, athwart the rivers Caesar's legions crossed.