The Jones Pulmonor was a bellow spirometer, and most of the inside was empty space.  This photo is of the back side of the spirometer.  If the front could be seen the box on the side was where a piece of pressure-sensitive graph paper was placed.  A pressure stylus would inscribe the volume-time curve from a spirometry maneuver.  The switch on the top controlled a motor that moved the graph paper during the test.  From an EBay listing.