Labourers were required to develop the resources of America and the West Indies, and to this end criminals under sentence of death were often pardoned on the understanding that they transported themselves to those colonies. Several convicts, however, were clever enough to secure the pardon and yet avoid fulfilment of the condition on which it was granted. By 1717 so many of these persons were at large, that arrangements were made[168] by which felons were to be kept in prison until they could be handed over to agents, who were required to give security that the undesirables in question were really deported. Fifty years later the practice of transportation was common, and had come to be esteemed as an easy and profitable means of getting rid, once and for all, of offenders caught transgressing the laws made by society for its protection. Though called transportation the system really amounted to perpetual slavery; it could nominally be inflicted for fourteen years, but was almost invariably for life. The convicts were handed over to contractors at so much a head, and shipped off to America to work on the plantations: many died on the voyage, thus reducing the profits of the traffic; in fact a Bristol contractor complained that if another plague broke out on his ship he would have to give up the business. To prevent this waste, an Act was passed in 1767[169] which provided that, for the future, contractors should take the convicts immediately they were sentenced, for fear that they should deteriorate during their sojourn in prison, and consequently fetch less money. After the American Revolution we lost this source of revenue, and penal establishments at Bermuda, Gibraltar, and in New South Wales took the place of the plantation in the social scheme. At the same time a change took place in the method of conveying convicts to their destination; they were no longer bought for the sake of the work that could be got out of them, but contractors were paid for carrying them to the penal establishments. The frightful mortality on board the ships continued, however, until the terms of the contract were altered; as soon as the practice of paying a fixed price for every man embarked was discontinued, and the payment of a larger sum for every man landed alive substituted, the convicts were treated more like human beings, and the death-rate on these voyages was no longer excessive.