P. 532, line 12

The equation should be:

$\frac{{\partial }^2{u}_{tr}}{\partial x^2}+\frac{{\partial }^2{u}_{ss}}{\partial x^2}+\frac{{\partial }^2{u}_{tr}}{\partial y^2}+\frac{{\partial }^2{u}_{ss}}{\partial y^2}=\frac{\partial u_{tr}}{\partial t}+\frac{\partial u_{ss}}{\partial t}.$

Return to errata.