1861 - The Okavango River
The first British edition was published in London in 1861 by Hurst & Blackett. Harper & Brothers published the first American edition in New York in 1861.
The Harper edition is preferable, for it has a map and the same number and quality of illustrations as the Hurst edition.
Below are images of the first British edition, which is difficult to find in original cloth binding in nice condition.
I sold off the copy that I once owned, but kept my copies of the Harper edition.