A quantity ζ which enters into equations at any point where the flow involves a change in volume, i.e. is dilatational. If the deformation is purely dilatational, the average of three normal stress components is: \[\sigma =\frac{1}{3}\ (\sigma _{xx}+\sigma _{yy}+\sigma _{zz})=- p+\zeta \ (\frac{\partial v_{x}}{\partial x}+\frac{\partial v_{y}}{\partial y}+\frac{\partial v_{z}}{\partial z})\] where p is the hydrostatic pressure at the point considered in the absence of motion.