From: Lectures on Dyspnea, Lecture III.  By Hyde Salter, MD.  The Lancet London: A Journal of British and Foreign Medicine,  Surgery, Obstetrics, Physiology, Chemistry, Pharmacology, Public Health and News, Volume 2, October 28, 1865, page 475.

“The principle of the action of the instrument is this:  All the inspired air id derived from cylinder A, (above this piston) and all the expired air passes into cylinder B; (from culinder B it is again discharged into the open air; thus none passes back or is breathed over again.  The act of inspiration draws air from cylinder A expels the respired air from cylinder B, and the act of expiration which breathes the expired air into cylinder B draws a fresh supply of unbreathed air into cylinder A.”