A proof & tralle hydrometer is used to calculate the alcohol volume in distilled spirits. The hydrometer is hollow and is weighted at the wider, bulb end. Dropped vertically into a test jar of spirits, the hydrometer will float, and the proof can be read where the surface of the spirit intersects the number scale. The higher the proof, the deeper the hydrometer will sink. On the reverse side is a tralle scale – its corresponding numbers are always half of the proof number, but I’ve yet to figure out what that measure means!
This hydrometer was meant to be used only for calculating distilled spirits, it would not accurately measure wine or beer. I’m told it has been in the family “forever”. (Some families hand down artworks or, you know, property…)