A stone is thrown from level ground at $16\ \text{m/s}$, at $53^\circ$ above the horizontal. Ignore air resistance, and take $g = 10\ \text{m/s}^2$, $\sin 53^\circ = 0.8$ and $\cos 53^\circ = 0.6$. At one instant during its flight, the stone's velocity is at right angles to its velocity of projection. What is its speed at that instant? Give the answer in $\text{m/s}$.