List of sources:
Barberton Scientific and Literary Society. Proceedings, 9 July 1888 to 28 October 1889. (Johannesburg Africana Library).
Cape of Good Hope. Report of the Meteorological Commission, 1882-1884.
De Kaap Annual, 1893, p. 33. Barberton: A.W. Bayly.
FamilySearch: William Cullen. https://www.familysearch.org/ark:/61903/3:1:3Q9M-CS4N-S2F4?view=index&personArk=/ark:/61903/1:1:QPRY-TW9N&action=view
General Directory of South Africa, 1890/1, directory of Barberton.
Gold Fields News (Barberton), 1 November 1888: "Altitude of Barberton".
Gunn, M. & Codd, L.E. Botanical exploration of southern Africa. Cape Town: Balkema, 1981.
Plug, C. Early scientific and professional societies in the Transvaal: Barberton 1887-1889. South African Journal of Cultural History, 1990, Vol. 4(3), pp. 190-199.