The total mileage of the United States expanded in ten years after 1840 from 2,800 to upwards of 9,000 miles of line. For some time not over four or five hundred miles annually had been constructed; but suddenly the new mileage laid down in 1848 jumped to more than fourteen hundred miles. This was a presage of the great expansion to occur in the next few years,--an expansion made possible partly as a result of important mechanical improvements and inventions. Notable among these was the substitution of the solid iron rail for the primitive method of plating beams with thin strips of iron. The manufacture of rails in the United States, begun in 1844, did much to stimulate the subsequent growth. The repeal of the law of 1832 permitting free entry of railway iron which took place in 1843, marks the beginning of a new era. During these eleven years almost five million dollars in duties on rails was refunded. The utmost activity in railroad building obtained from 1848 until the panic of 1857, interrupted only by a minor disturbance in 1854. The total mileage expanded more than threefold, attaining a total of 30,000 miles by 1860. A veritable construction mania prevailed in the states of Ohio, Indiana, and Illinois. Not very much, relatively, was accomplished in New York and Pennsylvania, and very little in New England, which was already well served. A dominant influence in promoting the new construction at this time was the imperative need of the South for foodstuffs. Cotton culture was in full swing in the lowlands of Alabama, Mississippi and Louisiana. An enormous steam and flat boat tonnage on the Ohio and Mississippi rivers had grown up to care for this trade.[7] By 1845 the river shipping amounted to nearly two million tons. Fifteen hundred out of four thousand steamboat arrivals at New Orleans in 1859, came from the Ohio river and the upper Mississippi. The vessels had also greatly increased in size. The flat boats which in 1820 carried only thirty tons of freight, were enlarged tenfold in tonnage and threefold in length by 1855, and in that year first began to be towed back up the river. A rapid increase in coal shipments down stream from Pittsburg also took place during the forties. From 737,000 bushels in 1844, to 22,000,000 bushels in 1855 and 37,900,000 in 1860, represents an enormous development of internal commerce. The lead mines of Missouri shipping through St. Louis had become important after 1832 and quadrupled in volume by 1848, attaining a total of 42,400,000 pigs of sixty pounds each. This traffic steadily dwindled, however, falling away by one half within the next ten years. Memphis was rapidly growing, outstripping the city of Natchez which had formerly played a more important part in the southern trade. But the most important element in this Mississippi river business was the shipment down stream of food stuffs. Produce received at New Orleans was valued at $26,000,000 in 1830, $50,000,000 in 1841, and $185,000,000 in 1860.