$(i)$ What causes the reverberation of thunder sound?
$(ii)$ $A$ $SONAR$ device on a submarine sends a signal and receives an echo $5 \, s$ later. Calculate the speed of sound in water if the distance of the object from the submarine is $3625 \, m$.

(N/A) $(i)$ Reverberation of thunder is caused by the successive and multiple reflections of sound waves from various reflecting surfaces,such as clouds and the Earth's surface.
$(ii)$ Given:
Distance of the object from the submarine $(d)$ $= 3625 \, m$
Total distance travelled by the signal $(2d)$ $= 2 \times 3625 \, m = 7250 \, m$
Time taken $(t)$ $= 5 \, s$
Speed of sound $(v)$ $= \text{Total Distance} / \text{Time}$
$v = 7250 / 5$
$v = 1450 \, m \, s^{-1}$
Therefore,the speed of sound in water is $1450 \, m \, s^{-1}$.