Breathing measurement. Physiological measurement of spiritualists' breathing during a seance. The spiritualists must maintain an air pressure to maintain the position of the floats in the U- shaped tube at centre. This measurement formed part of an experiment designed to discover whether voices produced during seances were caused by supernatural means or by ventriloquism by the spiritualists. Even when using this equipment, some spiritualists were still able to produce spirit voices". Photographed at the Society for Psychical Research in London, Great Britain, during the 1920s."