From: Proceedings of the Physiological Society, October 13, 1928.  Article “A new volume recorder” by RJS McDowall.

“The principal is an old one, being that of the spirometer and other similar apparatus with a fluid seal, but by improvement of the design friction is reduced to a minimum, while the curvature of the bell permits a large excursion of the lever.  The center of the curvature of the bell and of the weight is at the fulcrum, and if the center of gravity of the lever system is also exactly at the fulcrum the lever and bell are balanced in all positions, and for all practical purposes the entrance of equal amounts of air into the bell is recorded by equal excursions.  The apparatus cannot leak.  It is made by C.F.Palmer.”