Final answer:
A hydrometer is an instrument used for measuring the specific density of liquids, while a manometer is used to measure the pressure of a gas trapped in a container.
Step-by-step explanation:
A hydrometer is an instrument used for measuring the specific density of liquids based on the concept of buoyancy. It consists of a sealed hollow glass tube with a wider bottom portion for buoyancy, a ballast such as lead or mercury for stability, and a narrow stem with graduations for measuring.
To calibrate a hydrometer, it can be gently lowered into a liquid of known density and the reading on the scale at the air-liquid interface is recorded. This known reading can be used as a reference to measure the relative density of other liquids.
A manometer, on the other hand, is used to measure the pressure of a gas trapped in a container. It is a U-shaped tube partially filled with mercury. One end of the U-tube is connected to the gas chamber, and the other end is either closed or open to the atmosphere. The difference in the liquid levels in the two arms of the manometer is proportional to the pressure of the gas in the container.