The earliest beginnings of industrialisation can be traced to the development of the cotton industry in England in the first half of the 18th century. Cotton was imported from the United States to Liverpool and transported to Manchester and other Lancashire towns, where the first cotton mills were built. Rapid growth in industrialisation began in the late 18th century, the period often considered as the start of the Industrial Revolution. By the beginning of the 19th century, inventions were not just limited to the cotton industry. Steam engines were invented, providing a faster mode of transportation, instead of the use of horses and carriages. Steams engines and other machinery, of course, required coal as a fuel source, releasing the first emissions of carbon dioxide and other gases which led to declining air quality in many cities.