From ‘Physical and Natural Therapeutics: The Remedial Uses of Atmospheric Pressure, Climate, Heat and Cold, Hydrotherapeutic Measures, Mineral Waters, and Electricity’ by Georges Hayem, Lea, 1895, page 20.

“The open bottle is filled with water and then elevated so that the water flows into the second bottle up to the line marked zero. The stopcock is now closed, and the two bottles are placed on the same plane.  The patient takes an inspiration and applies his mouth to the funnel, opens the stopcock, and expires into the bottle.  At the end of the expiration the stopcock is again closed.  When the two bottle are placed so that the water in them is at the same level, the amount that is thereby made to flow into the graduated bottle indicates the volume of expired air at the pressure of the atmosphere.”