distribution of climatic zones in one of the past geological epochs point to an axis of rotation totally different from the present axis, there will be nothing left for the geophysicists but to accept this contention.” Schiaparelli,[1] in particular, has examined the question of polar displacement for the three cases of an entirely rigid earth, an entirely liquid earth, and one with a retarded adjustment to the position of the poles for the time being (i.e., a viscous earth). In the first case he comes naturally to the conclusion of Laplace of the invariability of the axis of rotation. On the other hand, in the case of the completely liquid earth, the poles are very movable, since then the flattening follows immediately every alteration of the position of the poles, and can therefore no longer contribute to the stabilization of the axis of inertia. Exceedingly rapid wanderings of the pole are to be expected in this case, such as do not occur in the earth’s history. Finally, on the third supposition, that of retarded adjustment, the earth behaves as a rigid body so long as the forces moving the poles do not exceed a certain value. Then there only exists Euler’s perturbation such as is observed to-day. But as soon as this limit is exceeded (which happens as soon as the radius of the curve of the perturbations exceeds the critical limit), the poles, in some sort, run away. Extensive, even if slow, polar wanderings are then able to take place. Now, since wanderings of the poles of this kind are manifestly detectable in the earth’s history, the conclusion must be drawn that the earth behaves as a viscous body.
The last phenomenon to be quoted as evidence of the viscosity of the earth is its oblateness. So far as the accuracy of our measurements permits us to form
- ↑ Schiaparelli, “De la rotation de la terre sous l’influence des actions géologiques” (Mém. près à l’observatoire de Poulkova à l’occasion de sa fête semiséculaire), pp. 1–32. Petrograd, 1889.
