A sensitive differential manometer used in the recording of small differential pressures is described. This instrument responds to pressure differences as small as 0.00003 cm of mercury and as rapid as 80 cycles per second. The moving element is a rectangular glass mirror 0.010 inch thick and approximately 0.060 inch×0.080 inch, supported on a glass shaft 0.007 inch in diameter. Motion is supplied by a thin rubber diaphragm approximately 0.003 inch thick. Included in recordings made with the aid of this instrument are wave forms originating in the human neck and finger, also pressure changes in a glass cell enclosing a tracheal breathing insect.