Hydrometer is based on the principle of floatation. The weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer

1. It is a direct reading instrument used for measuring the density or relative density of the liquid.
2. It consists of a cylindrical stem having a spherical bulb at its lower end, and a narrow tube at its upper end.
3. The lower spherical bulb is partially filled with leadshots or mercury. This helps hydrometer to float or stand vertically in liquids.
4. The narrow tube has marking so that relative density of a liquid can be read directly.
5. The liquid to be tested is poured into the glass jar. The hydrometer is gently lowered into the liquid until it floats freely.
6. The reading against the level of liquid touching the tube gives the relative density of the liquid.

1. The instrument used to measure atmospheric pressure is called Barometer.
2. It consists of a long glass tube which is closed at one end, open at the other filled with mercury and turned upside down in a container of mercury.
3. This is done by closing the open end of the mercury filled tube with the thumb and then opening it after immersing it in to a trough of mercury.
4. The Barometer works by balancing the mercury in the glass tube against the outer air pressure.
5. If the air pressure increases, it pushes more of the mercury up into the tub and if the air pressure decreases, more of the mercury drains from the tube.
6. The space between mercury and closed end is vacuum, since vacuum cannot exert any pressure, the level of mercury in the tube provides a precise measure of air pressure, which is called atmospheric pressure.
7. This type of instrument can be used in a lab or weather station.

The hole in the boat is below the water level. That means the boat is heavier. So it starts to sink because the hydrostatic pressure at the hole is always higher than the atmospheric pressure pushing down on the surface of the water in the boat. So the boat will sink.

When we put the ice cube in a glass of water it will float because density of ice cube is lesser than water. So it will float.
When we put the ice cube in a glass of alcohol it will sink because density of ice cube is greater than alcohol. So it will sink.
Density of water = 1 g cm^-3
Density of ice cube = 0.917 g cm^-3
Density of alcohol = 0.789 g cm^-3

P = hpg
Given Atmospheric pressure = 98.6 kPa
Density of mercury is = 13.6 x 103 kg m^-3
Acceleration due to gravity = 9.8 ms^-2
h = \(\frac { P }{ \rho g } \) = \(\frac { 98.6 \times {10}^{3}}{13.6\times{10}^{3}\times9.8}\) = 0.739 m ⇒ 740 mm