In the xy-plane, the region $y>0$  has a uniform field $B_{1}k$  and the region $\mathrm{y}<0$  has another uniform magnetic field $B_{2}k$ . A positively charged particle is projected from the origin along positive y-axis with speed ${\mathrm{\upsilon }}_0={\mathrm{\pi s}}^{-1}$ at $t=0$ , as shown in the figure. Neglect gravity in this problem. Let $\mathrm{t}=\mathrm{T}$ be the time when the particle crosses the x-axis from below for the first time. If ${\mathrm{B}}_2=4{\mathrm{B}}_1$  , the average speed of the particle, in ${\mathrm{ms}}^{-1}$ , along the x-axis in the time interval T is _______