The apparatus shown in figure is hydrometer

The hydrometer analysis is utilized for particle sizes finer than 75 μm. These particles pass through the last sieve (No. 200) of the Sieve Analysis.

A hydrometer is a device designed to measure the relative density of a liquid which refers to the ratio of the actual density of the substance to the density of the water. The apparatus consists of a cylindrical stem and a bulb that contains a specific portion of mercury or lead at the bottom, calibrated to float upright in the liquid. The liquid is poured in a tall cylinder usually made out of glass and the hydrometer is placed inside until it is stabilized. The test is based on the principle that in a low-density liquid, the hydrometer will sink deeper until it balances.

The hydrometer contains a scale which is used to record the relative density of the liquid based on its submersion.

The hydrometer grain size analysis takes advantage of the change in the relative density of a soil-water mixture as the soil particles sink. The test relies on the fact that when the soil is poured in the liquid, the relative density of the soil-water mixture will rise. As the soil particles sink the density decreases until it reaches the initial density of the liquid. The heaviest particles (larger in diameter) will sink first.