Dellinger, J. H.; Jolliffe, C. B. & Parkinson, T. Cooperative Measurements of Radio Fading in 1925, report, August 17, 1927; (https://digital.library.unt.edu/ark:/67531/metadc66491/m1/1/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .