A hydrometer is an instrument used to measure the density of liquids in comparison to water, also known as specific gravity. Hydrometers can be adjusted for different uses, such measuring the creaminess of milk (a lactometer), gauging the density of sugar in a liquid (a saccharometer), or determining the level of alcohol in spirits (alcoholmeter). Low-density liquids, such as alcohol, cause the hydrometer to sink deeper while higher-density liquids, such as milk, will not cause it to sink as far.

Martin Fisher, who made this example, operated a shop on 58 Chestnut Street in Philadelphia. He was well known philosophical instrument maker in the early 1800s. The set includes a brass hydrometer, a ruler made of ivory, a thermometer, and thirty-five circular brass weights.