- Correspondence and papers relating to aeronautics, 1904-1914. Much of the correspondence is with firms specialising in aeronautical equipment; there are other letters from H.R. Davies of Treborth, Bangor, who gave financial backing to W.E. Williams in the construction of a monoplane in 1910.
- Correspondence between Williams and Humber Ltd., relating to an invention for the improvement of the balancing of motorcar engines, May 1911.
- Correspondence between Williams and the agent of the Stewart-Dam Estate, Dunfanaghy, Co Donegal, regarding the silica content of sand found on the estate, 1914-15.
- Testimonials and references in support of Williams's applications for a university fellowship and for the post of Assistant Lecturer in Physics at UCNW, 1897-1906.
- Agreements between Williams and others regarding the prospecting and working of minerals on properties in Llandegai, Caernarfonshire, and Llanddeusant, Llanfflewin and Llanrhuddlad in Anglesey, 1909-1912. Also contains a report on the Cwmgraeanog and Marchlyn copper mine.
- Copies of articles by William Ellis Williams published in aeronautical journals: The Centre of Gravity (Aeronautics, December 1909); The Stresses on Aeroplane Framework (Aeronautics, July 1911); A New Method for the Measurement of Air Resistance on Planes and Curved Surfaces (Aeronautics, November 1911); The Pressure Distribution on an Aeroplane Wing in Flight (The Aeronautical Journal, October 1914) .