The reality of psychic phenomena : raps, levitations, etc.
A 1916 London study by W. J. Crawford, published by John M. Watkins and held in the Library of Congress Houdini Collection, treating spiritualist raps and levitations as physical phenomena to be measured. The four pictures are plain line diagrams of levers, pivots and force arrows, with curved beams rising from a base line and a labelled balance-arm sketch. A designer could take a clean, schematic vocabulary of arrows, labels and simple apparatus drawn in thin black line on cream paper.
