sicists, because they can prove that the earth is about two to three times as rigid at room-temperature as steel (which possesses a viscosity coefficient of 8 × 1011 c.g.s. units). We must consider this in a little more detail. The result is arrived at in three different ways. From the observations of Geiger and Gutenberg on the velocity of earthquake-waves in the core of the earth, it follows that the coefficient of viscosity at a depth of 0.4 radius of the earth is 36 × 1011 c.g.s., whilst for the zone of silicate rocks about 7 × 1011 c.g.s. is obtained. On the other hand, Schweydar[1] found from the elastic tides of the solid earth, which were measured by the horizontal pendulum, the value of the effective tidal rigidity of the earth 18 × 1011, for the centre of the earth 31 × 1011. Thirdly, a viscosity coefficient may also be calculated from the oscillations of the poles. These can be split into two overlapping periods, namely, a forced oscillation of the length of a year, which can be traced, according to Spitaler and Schweydar, to the action of the annual shifting of the atmospheric masses on the axis of inertia, with the chief phenomenon of a free swing of 14 months, which corresponds to a revolution of the rotation pole around the pole of inertia. Assuming a rigid earth, the period of this oscillation should only amount to ten months according to Euler’s theoretical calculations. Newcomb suspected that it may be extended by the yielding of the earth, which permits a partial adjustment of the form of the ellipsoid on the new direction of rotation. Hough and Schweydar calculated from this a coefficient of viscosity of 18 × 1011 in agreement with the result of the tidal observations. Schweydar
- ↑ W. Schweydar, Lotschwankung und Deformation der Erde durch Flutkräfte gemessen mit zwei Horizontalpendeln im Bergwerk in 189 m. Tiefe bei Freiberg i. Sa., Zentralbureau d. Internat. Erdmess., N.F., Nr. 38. Berlin, 1921.
