What device did Galileo invent to measure density?

Galileo invented the hydrostatic balance to measure density. He described the instrument in his 1586 treatise La Bilancetta, or The Little Balance, while he was still in his twenties.

The device compared the weight of an object in air with its apparent weight when suspended in water. From the difference, Galileo could determine the object's specific gravity and identify whether metals or alloys had been mixed or adulterated. This made the balance an important early example of quantitative experimental science.

The instrument is often confused with Galileo's telescope, but the telescope was an improved astronomical instrument he built after learning about Dutch designs in 1609. The hydrostatic balance served a different purpose: it investigated matter and density rather than the heavens. Galileo's work built on the buoyancy principle associated with Archimedes.