Kreigsmarine Hygrometer/Thermometer

Used on U-Boats in WWII to test the salinity of sea water. Glass instrument with a thermometer and weighted base. The thermometer card is marked Grade Des Hundertteiligen thermometers. The exterior of the glass is faintly marked with a Kreigsmarine eagle with M below and is complete in the original card case. In near mint condition, one wonders how many of these delicate instruments have survived. Measures 13" (33 cm)