hydrometer

noun

1. An instrument that floats in a liquid and measures its specific gravity on a scale.

   “[E]ach degree of the hydrometer, when at or near proof, is about a quart in a hundred gallons stronger or weaker, as the hydrometer vibrates under or over the silver speck or sight on the index or upper stem; at the intermediate or strong overproofs, is about a pint in ditto; and at the low underproofs is about three pints in ditto.”

   “On the wall were hanging thermometers, barometers, and hydrometers, and every other sort of ometer, numberless, dusty, and mysterious; […]”