Notably, the groups which he set up correspond in a number of cases to modern groups: Although Koelle's was not the first such study comparing different African languages,[1] (for example, a missionary called John Clarke had produced a similar work in 1848,[2] and still earlier Hannah Kilham had produced her Specimens of African Languages, Spoken in the Colony of Sierra Leone in 1828), yet in its accuracy and thoroughness it outclassed all the others and still proves useful today.
One area that was lacking was the Swahili coast of Kenya and Tanzania, since it seems that slaves from this region were generally taken northwards to Zanzibar and Arabia rather than southward towards America and Brazil.
This information, combined with a census of Sierra Leone conducted in 1848, has proved invaluable to historians researching the African slave trade in the 19th century.
[8] An analysis of the data shows that typically Koelle's informants were middle-aged or elderly men who had been living in Freetown for ten years or more.
[10] Included with a book is a map of Africa showing the approximate location, as far as it could be ascertained, of each language, prepared by the cartographer August Heinrich Petermann.
The orthography he eventually chose, after discussions in London, was not that of Karl Richard Lepsius (as is sometimes claimed), since it had not yet been published, but was based on a short document issued in 1848 by Henry Venn of the Church Missionary Society entitled Rules for Reducing Unwritten Languages to Alphabetical Writing in Roman Characters With Reference Specially to the Languages Spoken in Africa.
He retained seven of the eight vowels of Venn's system (i, e, ẹ, a, ọ, o, u, omitting ạ as in "but") but added length marks, a dot for nasalisation, and an accent to indicate the prominent syllable.
Koelle's language names are given in the left-hand column of the table below: some of the diacritics (such as the dot beneath ẹ and ọ, and the acute accent) have been omitted.