Hygrometer or hydrometer?

Do you know the difference between a hygrometer and a hydrometer? A hygrometer measures humidity, the amount of water vapour in air. A hydrometer, on the other hand, measures the density or specific gravity (SG) of a liquid by floating in the liquid. The word hygrometer is derived from the Greek hygros  meaning wet or moist, whereas hydro- comes from the Greek for water, hydro  (as in hydrated and hydrogen).