On 26th of September, 1786, a treaty of commerce and navigation between Great Britain and France[231] was concluded, whereby perfect freedom of navigation was mutually conceded; but before the expiration of twelve years, the stipulated period of its existence, one of the greatest political convulsions of modern times had overthrown the dynasty of the Bourbons, and France and England were again plunged into a more deadly war than had ever before been waged by these ancient and inveterate foes. England also formed treaties with Spain, Prussia, and Holland, and the treaty with Russia, which had been allowed to expire in 1786, was also then renewed. [Sidenote: Slave trade, and its profits.] Among numerous other matters, the slave-trade occupied a very considerable portion of the attention of Parliament and of the country during this period. By a return laid before Parliament in 1789, the number of slaves annually carried from the coast of Africa in British vessels was thirty-eight thousand; the number taken to the British West Indies, upon an average of four years, being estimated at twenty-two thousand five hundred.[232] Prior to the year 1760 no complete returns have been preserved of the number of ships thus employed; subsequently, however, it ranged from twenty-eight, measuring three thousand four hundred and seventy-five tons in the year 1761, to one hundred and ninety-two, measuring twenty thousand two hundred and ninety-six tons in 1776; but, during the war, from 1776 to 1783, this inhuman trade either languished, or the vessels formerly engaged in it were otherwise more profitably employed.[233] The majority of the slave-vessels were built expressly for the trade; and at the date of the report, 1786, from which we derive our information, six vessels were in course of construction, and about a dozen fitting out or ready for sea. A slave then cost from 8_l._ to 22_l._ in Africa, and realised in the West Indies from 28_l._ to 35_l._, the prices having about doubled during the previous century. In addition to the British import of slaves, the French carried away from Africa every year about twenty thousand; the Portuguese ten thousand; the Dutch four thousand; and the Danes two thousand; making a total of seventy-four thousand annually exported from Africa, including the thirty-eight thousand despatched in British vessels. A considerable portion of these found their way to the Spanish possessions. When after the restoration of peace with the colonies the London streets were filled with emancipated negroes, many of them, collected by an order of the 9th of December, 1786, were forwarded in British transports to Sierra Leone, where a settlement was formed.[234] [Sidenote: Trade between England and America and the West Indies re-opened.]