Patent number 392,711 by W.H.H. Barton.

“The instrument consists, essentially, of a flexible, collapsible, non-elastic reservoir or bag, preferably elongated or substantially cylindrical in shape, proved with a scale that indicates the cubic contents of the different portions of said bag when distended or filled.

“The bag or reservoir is provided at one end with a spindle or roll upon which the unfilled portion may be wound, so that the scale will indicate the cubic contents of the part of the reservoir that is filled.  The said spindle or roll preferably forms the inlet to the reservoir and is provided with a suitable mouthpiece. One important advantage to this kind of spirometer arises from the fact that their is no back-pressure of the atmosphere, and the instrument is very light, compact and inexpensive.”