The line next opened, and connecting the Western system of lakes and rivers with tide-water, was that extending from Boston to Ogdensburg, composed of distinct links, the last of which was completed during 1850. The third was the New York & Erie, which was opened on the 22d of April, 1851. The fourth in geographical order was the Pennsylvania, which was completed in 1852, although its mountain division was not opened till 1854. Previous to this time its summit was overcome by a series of inclined planes, with stationary engines, constructed by the State. The fifth great line, the Baltimore & Ohio, was opened in 1853 still farther south. The Tennessee River, a tributary of the Mississippi, was reached in 1850 by the Western & Atlantic Railroad of Georgia, and the Mississippi itself by the Memphis & Charleston Railroad in 1859. In the extreme north the Atlantic & St. Lawrence, now known as the Grand Trunk, was completed early in 1853. In 1858 the Virginia system was extended to a connection with the Memphis & Charleston and with the Nashville & Chattanooga Railroad. The eight great works named, connecting the interior with the seaboard, are the trunks or base-lines upon which is erected the vast system that now overspreads the whole country. They serve as outlets to the interior for its products, which would have little or no commercial value without improved highways, the cost of transportation over which does not equal one-tenth that over ordinary roads. The works named, assisted by the Erie Canal, now afford ample means for the expeditious and cheap transportation of produce-seeking Eastern markets, and could without being overtaxed transport the entire surplus products of the interior. Previous to 1850 by far the greater portion of railroads constructed were in the States bordering the Atlantic, and, as before remarked, were for the most part isolated lines, whose limited traffics were altogether local. Up to the date named, the internal commerce of the country was conducted almost entirely through _water_ lines, natural and artificial, and over ordinary highways. The period of the settlement of California marks really the commencement of the new era in the physical progress of the United States. The vast quantities of gold it produced imparted new life and activity to every portion of the Union, particularly the Western States, the people of which, at the commencement of 1850, were thoroughly aroused as to the value and importance of railroads. Each presented great facilities for the construction of such works which promised to be almost equally productive. Enterprises were undertaken and speedily executed which have literally converted them into a network of lines, and secured their advantages to almost every farmer and producer.