Consider the network for a two-dimensional system without internal volumetric generation having nodal temperatures shown below. If the grid spacing is $125 \mathrm{~mm}$ and the thermal conductivity of the material is $50 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}$, calculate the heat rate per unit length normal to the page from the isothermal surface $\left(T_{s}\right)$.