How did John Hanning Speke and James Grant provide evidence that Lake Victoria fed the Nile?

John Hanning Speke and James Grant provided evidence that Lake Victoria was a source of the Nile through a second expedition beginning in 1860, sponsored by the Royal Geographical Society.12

During this journey, the pair traveled through the region west of the lake, where they discovered the Kagera River and observed the Virunga Mountains. Their exploration culminated on July 28, 1862, when Speke reached the lake's exit point and identified the Ripon Falls. Upon witnessing the water flowing out of the lake, Speke concluded that the Nile undoubtedly rose in the lake, which he had previously named in honor of Queen Victoria. Following this discovery, Speke and Grant traveled northward along the river's course toward Gondokoro, further substantiating their findings.21