While in England, Livingstone was persuaded to explore south of Lake Victoria to settle the "last geographical puzzle" about the Nile headwaters. As a byproduct of his missionary travels, he did determine that the largest lake in Africa was, indeed, the remotest source of the Nile and did not connect with Lake Tanganyika.
Harper's Magazine in its article included many references by Livingstone to the slave trade during his African travels:
"Today we passed the body of a dead woman tied by her neck to a tree. The people of the country explained that she had been unable to keep up with the other slaves in a gang. Her master had determined that she should not recover to be the property of any one else.
"Others tied in a similar manner were seen farther on. There is a double purpose in these murders. The terror inspired in the minds of survivors spur them on to endure the hardships of the march.
"One day, one of my men wandered from the party and came upon a number of slaves yoked together with slave-sticks. They had sickened for want of food and had been left to die. When found, they were too weak to speak. Some were mere children.
"Our march for a part of the way was through a depopulated country. The natives differ as to the cause. Some say slave wars and assert that the Makon from the vicinity of Mozambique played an important part."
On another occasion, Livingstone wrote: " A company of slaves passed, singing as if they did not feel the weight and degradation of the slave-stick about their necks. Upon asking the cause of their mirth, I was told that they were rejoicing at the idea of coming back after death and haunting and killing those who had sold them into slavery.
"Their song ran, ?˜Oh, you sent me off to Manga (on the sea coast), but the yoke is off when I die. Back I shall come to haunt and kill you.' Then all joined in the chorus, in which the name of each trader was repeated. It told not of mirth, but of the bitterness and tears of the oppressed."
Upon reaching the country west of Tanganyika, Livingstone noted: "The Arabs have an inkling of the vast quantities of ivory that might be procured here. Those hordes of Uijijian traders, in all probability, will eventually destroy tribe after tribe by slave-trading and pillage -- as they have in so many other regions."