A much later, auxiliary reference to the last. Here the foreign observer is on the Gabon River. RH Milligan writes this in THE JUNGLE FOLK OF AFRICA, 1908.

Their singing, like their instrumental music, has not much "tune" to it, but there is always a stirring rhythm and a certain weird and touching quality, which impressed me the more because I could never quite understand it,—the same elusive charm that characterizes the singing of the negroes of the Southern States. I do not refer to the negro songs composed by white men, which are entirely different, but the melodies that the negro sings at his work. The native songs are of the nature of chants, and turn upon several notes of a minor scale. But it is not quite our minor scale. There is one prominent and characteristic note, which I confess defied me, though it may have been a minor third slightly flat. I found it very difficult to reduce their songs to musical notation.

He seems to refer to "blue" notes. Also raises the issue of what much earlier observers meant when they described "plaintive minor melodies" -- and the issue of interpreting their musical notations nowadays.

The words of most of the songs are improvised by the leading voice, and have a regular refrain in which all join. But if they wish to sing in chorus, as in their dance-songs, any words will serve the purpose and the same sentence may be repeated for an hour. "Our old cow she crossed the road " were luminous with propriety and sentiment in comparison with the words that they will sometimes sing in endless repetition. " The leopard caught the monkey's tail," "The roots grow underneath the ground," are samples of their songs. Their canoesongs I like best of all. The rhythm is appropriate and one almost hears the sound of the paddles. They sing nearly all the time as they use the paddle or the oar, and on a long journey they say it makes the hard work easier. If they should take a white man on a journey and, not being his regular workmen, should expect a "dash"—a fee, or present, in African vernacular—the leading voice will sing the white man's praises on the journey, alluding in particular to his benevolence, while the others all respond, seeking thus by barefaced flattery 'and good-natured importunity to shame the meanness out of him.