railroad lines were short haul project designed to move cotton to the nearest river or ocean port. Meanwhile, steamboats provided a good transportation system on the inland rivers. With the use of interchangeable parts popularized by Eli Whitney, the factory system began in which workers assembled at one location to produce goods. The early textile factories such as the Lowell mills employed mainly women, but generally factories were a male domain. By 1860, 16% of Americans lived in cities with 2500 or more people; a third of the nation's income came from manufacturing. Urbanized industry was limited primarily to the Northeast; cotton cloth production was the leading industry, with the manufacture of shoes, woolen clothing, and machinery also expanding. Energy was provided in most cases by water power from the rivers, but steam engines were being introduced to factories as well. By 1860, the railroads had made a transition from use of local wood supplies to coal for their locomotives. Pennsylvania became the center of the coal industry. Many, if not most, of the factory workers and miners were recent immigrants from Europe, or their children. Throughout the North, and in southern cities, entrepreneurs were setting up factories, mines, mills, banks, stores, and other business operations. In the great majority of cases, these were relatively small, locally owned, and locally operated enterprises. Immigration and Labor To fill the new factory jobs, immigrants poured into the United States in the first mass wave of immigration in the 1840s and 1850s. Known as the period of old immigration, this time saw 4.2 million immigrants come into the United States raising the overall population by 20 million people. Historians often describe this as a time of "push-pull" immigration. People who were "pushed" to the United States immigrated because of poor conditions back home that made survival dubious while immigrants who were "pulled" came from stable environments to find greater economic success. One group who was "pushed" to the United States was the Irish, who were attempting to escape the Great Famine in their nation. Settling around the coastal cities of Boston, Massachusetts and New York City, the Irish were not initially welcomed because of their poverty and Roman Catholic beliefs. They lived in crowded, filthy neighborhoods and performed low-paying and physically demanding jobs. The Catholic Church was widely distrusted by many Americans as a symbol of European autocracy. German immigration, on the other