From: The cult of the sound body. Apparatus employed in modern physiological laboratories to test physical development. By T.V. Davidson.  Scientific American Monthly.  Jan 1921, page 34.

“To discover the amount of air taken into the lungs during a deep inspiration, a very simple spirometer is used at Joinville.  It consists of a bell glass balanced by a counter-weight and immersed in a cylinder three quarters full of water.  The subject exhales the air through a rubber tube with a glass mouthpiece, which runs to the lower tubulur of the spirometric vessel.  The section of this tube is exactly equal to that of the trachea so that there will be no resistance to the air proceeding from the lungs of the subject and no alteration of the respiratory rhythm.  The air exhaled causes the internal pressure to be increased and the amount of the increase is shown on a manometer.  Furthermore, if care is taken to graduate the manometer beforehand by injecting exactly 1, 2, 3, 4, 5, etc, liters of air and marking the corresponding level of water for each, the volume of air injected can be read off at once.”