Before the death of Granville Sharp, Clarkson had already turned his attention to the question of Negro Slavery. He had even selected it for the subject of a college Essay; and his mind became so possessed by it that he could not shake it off. The spot is pointed out near Wade’s Mill, in Hertfordshire, where, alighting from his horse one day, he sat down disconsolate on the turf by the road side, and after long thinking, determined to devote himself wholly to the work. He translated his Essay from Latin into English, added fresh illustrations, and published it. Then fellow labourers gathered round him. The Society for Abolishing the Slave Trade, unknown to him, had already been formed, and when he heard of it he joined it. He sacrificed all his prospects in life to prosecute this cause. Wilberforce was selected to lead in parliament; but upon Clarkson chiefly devolved the labour of collecting and arranging the immense mass of evidence offered in support of the abolition. A remarkable instance of Clarkson’s sleuth-hound sort of perseverance may be mentioned. The abettors of slavery, in the course of their defence of the system, maintained that only such negroes as were captured in battle were sold as slaves, and if not so sold, then they were reserved for a still more frightful doom in their own country. Clarkson knew of the slave-hunts conducted by the slave-traders, but had no witnesses to prove it. Where was one to be found? Accidentally, a gentleman whom he met on one of his journeys informed him of a young sailor, in whose company he had been about a year before, who had been actually engaged in one of such slave-hunting expeditions. The gentleman did not know his name, and could but indefinitely describe his person. He did not know where he was, further than that he belonged to a ship of war in ordinary, but at what port he could not tell. With this mere glimmering of information, Clarkson determined to produce this man as a witness. He visited personally all the seaport towns where ships in ordinary lay; boarded and examined every ship without success, until he came to the very _last_ port, and found the young man, his prize, in the very _last_ ship that remained to be visited. The young man proved to be one of his most valuable and effective witnesses. During several years Clarkson conducted a correspondence with upwards of four hundred persons, travelling more than thirty-five thousand miles during the same time in search of evidence. He was at length disabled and exhausted by illness, brought on by his continuous exertions; but he was not borne from the field until his zeal had fully awakened the public mind, and excited the ardent sympathies of all good men on behalf of the slave.