Set of six small glass spheres with "peduncles," used as hydrometers. They are illustrated in the manuscript Diari [Diaries] of the Cimento academicians in connection with an experiment on the density of water, performed on June 19, 1657. The sphere was placed in equilibrium in a cylindrical vase filled with water. The density was then increased by dissolving salt in the water, causing the sphere to rise. Adding ashes to the water augmented the upward thrust on the sphere.