At first I thought this antique Griebel Instrument Co. hydrometer was an old thermometer, until I noted that the numbers on the side run 0 to 6 rather than showing temperatures. I’ve since learned that a hydrometer is used for measuring the relative density of liquids based on buoyancy: you pour liquid into a container, lower the hydrometer into it until it floats, and then the point at which the surface of the liquid touches the stem of the hydrometer gives a measure of its relative density. This one, which contains mercury—making one very careful handling it!— dates to sometime between 1900-1920, comes in a great slide-top long wooden box with dovetailed corners and the original cotton batting inside, and I think it quite a fantastic looking instrument!
Hydrometer measures 7 1/4" long x 3/4" wide. Box measures 10" long x 1 3/4" wide and deep (and contains original--pretty dusty--cotton batting.