Early Balloon Statoscope instrument. (see note)

The statoscope is used for indicating small changes of altitude, It was important for balloonists to know whether their balloon was rising or sinking to avoid the waste of gas or ballast. Air was trapped in the statoscope by pinching a rubber tube attached to it. If the balloon rose the enclosed air would expand because of the decrease in external pressure. If the balloon descended the opposite would happen. This moved a rubber membrane in the instrument which caused the pointer to move.