From: Spirometer measurement of oxygen consumption by the rebreathing method. By Charles Claude Guthrie, Archives of Internal Medicine, Volume 28, 1922, page 843.

“The tap (Fig. 2) serves to connect the spirometer through the absorber with the pump, the subject or the outside air.  The rotating portion of the tap is hollow and provided with two openings spaced 90 degrees apart.

“In the first position of the tap, the absorber is shut off and the subject breathes room air.  Position two is a quarter turn (clockwise, i.e. to the right) and switches the subject to the absorber and thus to the air in the spirometer, at the same time closeing the opening to the outside.  Position three connects the absorber with the pump, and position four connects the pump to the outside air.”