on the air inside the drum is transmitted from the drum through an air-tight tube to a little pencil which it drops and lifts. The pencil, as it rises and falls, touching always a sheet of smoked paper traveling over a cylinder on the recording device, traces a line whose rising strokes represent accurately the drawing of air into the chest and whose falling represents its expulsion.
It was clear to Trant that the professor's rapid explanation, though plain enough to the psychologists already familiar with the device, was only partly understood by the big men. It had not been explained to them that changes in the breathing so slight as to be imperceptible to the eye would be recorded unmistakably by the moving pencil.
Professor Schmalz turned to the second instrument. This was a plethysmograph, designed to measure the increase or decrease of the size of one finger of a person under examination as the blood supply to that finger becomes greater or less. It consists primarily of a small cylinder so constructed that it can be fitted over the finger and made air-tight. Increase or decrease of the size of the finger then increases or decreases the air pressure inside the cylinder. These changes in the air pressure are transmitted through an air-tight tube to a delicate piston which moves a pencil and makes a line upon the record sheet just under that made by the pneumograph. The upward or downward trend of this line shows the increase or decrease of the blood supply, while the smaller vibrations up and down record the pulse beat in the finger.
There was still a third pencil touching the record
