Through a long preliminary period the principal center of interest was the Potomac Valley, towards whose strategic head Virginia and Maryland, by river-improvement and road-building, were directing their commercial routes in amiable rivalry for the conquest of the Western trade. Suddenly out from the southern region of the Middle Atlantic States went the Cumberland National Road to the Ohio. New York instantly, in her zone, took up the challenge and thrust her great Erie Canal across to the Great Lakes. In rapid succession, Pennsylvania and Maryland and Virginia, eager not to be outdone in winning the struggle for Western trade, sent their canals into the Alleghanies toward the Ohio. It soon developed, however, that Baltimore, both powerful and ambitious, was seriously handicapped. In order to retain her commanding position as the metropolis of Western trade she was compelled to resort to a new and untried method of transportation which marks an era in American history. It seems plain that the Southern rivals of New York City--Philadelphia, Baltimore, and Alexandria--had relied for a while on the deterring effect of a host of critics who warned all men that a canal of such proportions as the Erie was not practicable, that no State could bear the financial drain which its construction would involve, that theories which had proved practical on a small scale would fail in so large an undertaking, that the canal would be clogged by floods or frozen up for half of each year, and that commerce would ignore artificial courses and cling to natural channels. But the answer of the Empire State to her rivals was the homely but triumphant cry "Low Bridge!"--the warning to passengers on the decks of canal boats as they approached the numerous bridges which spanned the route. When this cry passed into a byword it afforded positive proof that the Erie Canal traffic was firmly established. The words rang in the counting-houses of Philadelphia and out and along the Lancaster and the Philadelphia-Pittsburgh turnpikes--"Low Bridge! Low Bridge!" Pennsylvania had granted, it has been pointed out, that her Southern neighbors might have their share of the Ohio Valley trade but maintained that the splendid commerce of the Great Lakes was her own peculiar heritage. Men of Baltimore who had dominated the energetic policy of stone-road building in their State heard this alarming challenge from the North. The echo ran "Low Bridge!" in the poor decaying locks of the Potomac Company where, according to the committee once appointed to examine that enterprise, flood-tides "gave the only navigation that was enjoyed." Were their efforts to keep the Chesapeake metropolis in the lead to be set at naught?