In 1717, the Parliament of Great Britain passed the Transportation Act 1717, which allowed for the penal transportation of tens of thousands of convicts to North America, in order to alleviate overcrowding in British prisons. By the time penal transportation ceased during the American Revolutionary War (1775–1783), some 50,000 people had been transported to the New World under the law. When the American market closed to them, the convicts were then sent to Australia. In total, 300,000 to 400,000 people were shipped to the North American colonies as unfree laborers, between 1/2 and 2/3 of all white immigrants.