The Nuttall Encyclopedia
A voluminous heterogeneous collection of treatises connected with the Vedas, and the chief source of our knowledge of the early metaphysical speculations and ethical doctrines of the Hindus; they are to a great extent apocryphal, and are posterior to the rise of Buddhism.
Wood, James, ed. Entry for 'Upan`ishads'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/u/upanishads.html. Frederick Warne & Co Ltd. London. 1900.