RECENTLY, Dr. Richard K. Cook1 has criticized as errors certain equations in my book2 in which mixed units occur. In particular, he points to my Eq. (1), which expresses the energy stored in a crystal upon which an arbitrary mechanical strain and an arbitrary electric field are impressed. His assertion that "the reader is therefore obliged to examine closely the origin of the formulae he uses" would be more convincing if he had cited instances in which an incorrect conclusion has been drawn from Eq. (1). This equation is : where the c's are elastic constants at constant field, the S's are strain-components, η" is the dielectric susceptibility of the clamped crystal, E the field-strength, e a piezo-electric stress-constant, J is the mechanical equivalent of heat in ergs per calorie, is the density, C the specific heat in cal.gm.-1 deg.-1, T the absolute temperature, θ = ΔT is a small change in temperature, q is a thermo-elastic constant, and p a pyro-electric constant.