In 1811 and 1812, Fulton built two steam ferry-boats for the North River, and soon after added a third for the East River. These boats were the beginning of the magnificent steam ferry system which is to-day one of the chief wonders of New York. They were what are called twin-boats, each of them consisting of two complete hulls, united by a deck or bridge. They were sharp at both ends, and moved equally well with either end foremost, so that they could cross and re-cross without being turned around. These boats were given engines of sufficient power to enable them to overcome the force of strong ebb tides; and in order to facilitate their landing, Fulton contrived a species of floating dock, and a means of decreasing the shock caused by the striking of the boat against the dock. These boats could accommodate eight four-wheel carriages, twenty-nine horses, and four hundred passengers. Their average time across the North River, a mile and a half wide, was twenty minutes. The introduction of the steamboat gave a powerful impetus to the internal commerce of the Union. It opened to navigation many important rivers (whose swift currents had closed them to sailing craft), and made rapid and easy communication between the most distant parts of the country practicable. The public soon began to appreciate this, and orders came in rapidly for steamboats for various parts of the country. Fulton executed these as fast as possible, and among the number several for boats for the Ohio and Mississippi Rivers.