The terrible revolt of the slaves in the West Indies, beginning in 1781, gave the growing anti-slavery sentiment an immense impetus. It also gave the slave owners pause. The cotton-gin had not yet been invented. Slavery was on a shifty economic basis in the South. Great Britain passed the first law to limit the slave trade in 1788; the United States outlawed the trade in 1794. In 1824 Great Britain declared the slave trade piracy. During these years, and during the years that followed, until the last slaver left New York Harbor in 1863, the trade continued under the American flag, in swift, specially constructed American-built ships. As the restrictions upon the trade became more severe in the face of an increasing demand for slaves, "the fitting out of slavers developed into a flourishing business in the United States, and centered in New York City." _The New York Journal of Commerce_ notes in 1857 that "down-town merchants of wealth and respectability are extensively engaged in buying and selling African Negroes, and have been, with comparatively little interruption for an indefinite number of years." A writer in the _Continental Monthly_ for January, 1862, says:--"The city of New York has been until of late the principal port of the world for this infamous commerce; although the cities of Boston and Portland, are only second to her in distinction." During the years 1859-1860 eighty-five slavers are reported to have fitted out in New York Harbor and these ships alone had a capacity to transport from 30,000 to 60,000 slaves a year.[20] The merchants of the North pursued the slave trade so relentlessly because it paid such enormous profits on the capital outlay. Some of the voyages went wrong, but the trade, on the whole, netted immense returns. At the end of the eighteenth century a good ship, fitted to carry from 300 to 400 slaves, could be built for about $35,000. Such a ship would make a clear profit of from $30,000 to $100,000 in a single voyage. Some of them made as many as five voyages before they became so foul that they had to be abandoned.[21] While some voyages were less profitable than others, there was no avenue of international trade that offered more alluring possibilities. Sanctioned by potentates, blessed by the church, and surrounded with the garments of respectability, the slave trade grew, until, in the words of Samuel Hopkins (1787), "The trade in human species has been the first wheel of commerce in Newport, on which every other movement in business has depended.... By it the inhabitants have gotten most of their wealth and riches." (Spears, p. 20.) After the vigorous measures taken by the British Government for its suppression, the slave trade was carried on chiefly in American-built ships; officered by American citizens; backed by American capital, and under the American flag.