From “On respiration in singing” by Joseph Joal, Published by F.J. Rebman, 1895, page 84.
“…this instrument has a double Mariotte’s vase, composed of two chambers, placed over one another and separated by two semi-circular diaphragms, F H, which are joined by a vertical septum. The two chambers are put into communication by tube I. The apparatus being half full of water, and the liquid filling the upper reservoir, if we breathe by the mouthpiece A, a quantity of water is displaced, and flows out of the upper reservoir, into the lower one; the graduated glass permitting us to determine the level of the liquid.
“When the upper chamber is empty it suffices to turn the instrument upside down, corking the tube E, and placing the mouthpiece on the chamber D, at the same time that we raise the screw C, and close the opening C.”