From Patent application number 471,289 dated March 22, 1892.
“The operation of my invention is as follows: The mouth-piece a is placed in the mouth and air slowly inhaled until the lungs are completely filled. The opening S in the side of the main tube is closed by a finger of the person using the instrument and the air which has been taken into the lungs is forced out. This outward passage of air operates the valve B to close the outlet b, and the column of air, acting on the piston or plunger G, causes the same to move longitudinally in the tube E. A person can thus by repeated tests inform himself as to the increase in the capacity of the lungs by the distance the plunger is moved at each exhalation. When the instrument is removed from the mouth, the air withing the tube is forced out through the mouth-piece and the opening S by reason of the spring K drawing the plunger or piston back to its normal position.”