What is the pressure on a swimmer $20 \, m$ below the surface of water in $atm$?

$A$ sudden drop in the mercury level by $10 \,mm$ or more is a sign of ..........

Mention two instruments which measure the atmospheric pressure.

$A$ barometer tube reads $75 \,cm$ of $Hg$. If the tube is gradually inclined at an angle of $30^{\circ}$ with the horizontal,keeping the open end in the mercury container,then find the length of the mercury column in the barometer tube in $cm$.

The height of a mercury barometer is $75 \ cm$ at sea level and $50 \ cm$ at the top of a hill. The ratio of the density of mercury to that of air is $10^4$. The height of the hill is ....... $km$.

$A$ bubble of volume $V_0$ is released from a depth $h$ in a lake. If the atmospheric pressure is $P$ and the temperature is assumed to be constant,what is the new volume of the bubble when it reaches the surface? (Density of water is $\rho$)