From “The respiratory mechanism in pneumonia”, by Newburgh LH, Means JH, Porter WT, The Journal of Experimental Medicine, Volume 24, page 583, 1916. The spirometer is a Krogh type.
“A portion of the apparatus employed is shown in Text-Fig. 1. The tracheal canulla of the animal in which the respiratory reaction was to be measured was joined to a rubber tube placed between two Tissot valves connected in such a way that the animal breathed into a spirometer and out of a bottle connected in turn with the spirometer, so that the lungs, the spirometer, the bottle and the connecting tubes formed a closed system. Evidently by this arrangement the volume of air passing into and out of the chest was recorded by the spirometer, while the carbon dioxide exhaled by the animal constantly accumulated in the closed system. Samples of the air thus enriched with carbon dioxide were withdrawn at frequent intervals by the mercury tubes shown in Text-fig. 1. As each sample was taken, a mark was made on the spirometer record (the electric time-signal is not shown in Text-fig. 1). As the carbon dioxide in the respired air increased, the spirometer curve became deeper, until the maximum was reached.”