Hygrometer of the type invented or perfected by Vincenzo Viviani. The hygroscopic substance is a paper ribbon. The brass bar has a small turned column at each end. The columns carry two small rolls on which the ends of the paper ribbon are wrapped. The ribbon is weighted at the center by a wooden staff carrying a small pointer (the ribbon and staff are modern restorations). The changes in atmospheric humidity cause variations in the length of the paper ribbon, which are registered on a scale. This instrument is identical, even in size, to item inv. 3, 2437.