Currents of European expansion had lapped the shores of Africa since the fifteenth century. But at the end of the eighteenth century the continent and its peoples were still little known. If anything, European knowledge of and interest in Africa had declined since the heyday of Portuguese discovery and expansion in the fifteenth and sixteenth centuries. The coastline was tolerably well known, even if its scientific survey was to be mainly the work of the early nineteenth century, and if the east coast was now little frequented by European mariners. But what knowledge of the interior the Portuguese had once gained had been but indifferently passed on to, or remembered by, the other Europeans who had now surpassed them as builders of empires overseas. Following courses set by forgotten Portuguese embassies, French merchant-explorers had sought to make the Senegal river a highway to the empires and gold-mines of the western Sudan that were known partially through some (though not always the most accurate) of the medieval Arab writers. But their ambitions had been frustrated, in part by lack of consistent commercial backing in France, in part by the hostility both of the Sudanese peoples and of British sea power. In Guinea, Europeans had been content with their trade in Negro slaves for the Americas. Slaves were readily purchasable from African merchants and rulers at the coast. Thus Europeans lacked any incentive to penetrate inland, while at the same time established African interests existed to block any such penetration. Further south, the African kingdoms of the lower Congo and Angola, and the protectorates which Portugal had once sought to establish over them, had both been largely destroyed by the concentration of Portuguese merchants on this same trade.