Large profits were made in the trade, which had its centre at Lagos, and by the labour of the captives the great estates of southern Portugal were speedily brought under cultivation. The employment of slave labour was to have serious consequences in the near future; but at this period, during the life of Dom Henry, it had not yet begun to drive the poorer class of the Portuguese people out of work in the fields, and into more precarious modes of earning a livelihood. It is not necessary to do more than notice the commencement of the slave trade here; it is far more important to trace the progress of the explorers.[10] In 1445 Nuno Tristão sailed as far as the Senegal river, and in the same year, Diniz Dias, his most daring rival, discovered Guinea, and first saw the really black negroes. This advance, as a glance at the map will show, meant much; the Portuguese explorers had now thoroughly learnt how to find their way round the inhospitable shoulder of North-west Africa; Cape Bojador and Cabo Branco had no terrors for them, and their hopes of reaching India were excited by finding that the coast trended abruptly to the east. The country, too, was very different to that which they had toiled around so slowly; the fertile land of Guinea with its powerful negroes, its spices and ivory, and its prospect of gold, gave them encouragement, and on their return, the acute merchants of Lisbon were not long in opening up a trade with the newly-discovered country. Unfortunately the slave trade accompanied the ventures of the Lisbon merchants, and the white men, instead of making friends with the blacks, did not hesitate to seize them and to sell them into slavery. The Church made no effort to restrain this traffic; the blacks were heathen, and so it was to their advantage to be brought to a Christian land to work, and perhaps to be converted. The next two years were marked by the greatest activity. In 1446 Diniz Diaz reached Cape Verde, which he called by that name from its green appearance; and in the same year, Nuno Tristam was killed in a chase after slaves, and Alvaro Fernandes, the nephew of João Gonçalves Zarco, who had discovered Madeira, starting from that island, went one hundred leagues further than Cape Verde, and left João Fernandes at his own request among the negroes. It is a strange commentary upon the death of Nuno Tristão, that João Fernandes was able to remain among the negroes for seven months in safety, learning their language and studying their customs. It shows that there was no deep-rooted antipathy between the whites and the blacks, and that the latter only attacked the Christians, when they showed themselves enemies, and tried to rob them of their liberty. João Fernandes was taken off in safety by Antam Gonçalves, in the year 1447, and testified to Prince Henry that the blacks, if heathen, were not monsters, but people of peaceful and affectionate dispositions.