Question of a tricritical point in a compressible Ising model

We point out that a compressible Ising model with antishear forces, which was recently introduced by Baker and Essam, has a finite tricritical pressure $P_t$ in the Y approximation. We show that the first-order transition and its associated instability discussed by Baker and Essam can be understood on the basis of Le Châtelier's principle but occurs only under a constraint of constant external force. Thus, for $P>\mathrm{}{P}_t$, the renormalized exponents of the continuous transition are realizable. Homogeneous dilatations are properly accounted for; however, effects due to boundary conditions cannot be ruled out. Finally we show that at high enough pressures the system is unstable with respect to uniform shear deformation and discuss this result.