Mosso Plethysmograph
This contraption, which dates from around 1880, was designed to measure tiny changes in the human body, including blood pressure. The patient inserted a forearm into a glass tube that floated in an alcohol-and-water mixture. Blood flow made the arm expand, pushing some of the water out and making the tube float lower, raising the counterweight and a stylus, which marked its position on a revolving-drum kymograph (not pictured). Thus, the doctor could “read” a patient’s blood pressure.