From Artificial Respiration in Man, by Prof. E. A. Schafer, The Harvey Lectures, Volume 3, Academic Press, 1909, page 226.

“Hutchinson’s spirometer is defective on one point, viz., that the inner cylinder is not equally balanced at all points of its immersion in the water which is contained in the outer cylinder.  This defect was made good by Marcet, by the introduction of a secondary counterpoise.  This works from a cam-like projection attached to the pulley over which the cord carrying the main counterpoise passes. This cam is so arranged that the secondary counterpoise exerts more leverage the more the inner cylinder of the spirometer becomes lifted out of the water as air passes into it; and in this manner the increased weight of the cylinder is, more or less exactly, compensated.”