Professor Haefeli’s beautiful results are interesting both in themselves and as a corollary to our own measurements in the field Reference Gerrard, Perutz and Roch ¹ and in the laboratory.Reference Glen ² We have found in both cases a rapid increase in the rate of shearing as a function of increasing stress, somewhat like that shown in the full curve of Haefeli’s Fig. 1 (p. 95). In the field experiment the ice was at the pressure melting point, whereas in the laboratory pressure melting was excluded by carrying out the experiments at −1.5° C. Nevertheless, the dependence of the rate of shearing on shear stress was similar in the two cases, and possibly differed by no more than a scale factor due to the different temperatures at which the two experiments were done.

Haefeli rightly stresses the importance of distinguishing between the intrinsic effect of the shear component of the stress and the effect of hydrostatic pressure, and suggests that the latter should not be left out of consideration. Our experience rather seems to indicate that the first factor is overwhelming in cases like the one investigated by Haefeli, and that thermo-dynamic effects, i.e. pressure melting leading to the presence of a liquid phase at the grain boundaries, may only have a secondary influence.

In conclusion, we should like to make a plea that the time has come to abandon the term “viscosity” in discussions of glacier flow, because it has little value in relation to plastic materials showing the kind of behaviour that ice does.Reference Glen ² In cases where complicated stress systems are involved, such as the closing of a tunnel or the penetration of a ball, a curve of deformation rate against depth or load might be more informative and be more easily applied to other cases.