the case with impulses of short duration, as with the quickly alternating tides, or still more earthquake-waves, and perhaps also the oscillations of the poles. But as soon as we have to deal with thousands or millions of years instead of seconds, days or years, we must say: “the earth has only the viscosity of steel; it behaves, therefore, as a viscous, fluid body.”
It cannot be in any way denied that these ideas are somewhat paradoxical. But it must not be forgotten that even experiments in the laboratory with viscous, fluid substances appear paradoxical, because they are contrary to usual experience. Pitch, for example, behaves as an absolutely solid body when subjected to blows and percussion, but, given time, it begins to flow under the influence of gravity; a piece of cork cannot be forced through a sheet of pitch, but after a lengthy period its slight buoyancy is sufficient to allow it to rise slowly through the pitch from the bottom of a vessel. Because these things appear paradoxical, the explanation of the flow of glaciers at first presented difficulties, so that special causes, as, for example, regelation (secondary freezing), were thought to be necessary, until the recent observation on the similar flowing polar glaciers, with their low internal temperatures, gave a more correct idea of the viscous fluidity of these objects.
There remains to be mentioned the fact that there are a great number of differently defined coefficients of viscosity, solidity and rigidity. Without going into the matter further, only one example will be given to show what properties of bodies are concerned.
Maxwell calls a body soft if it reacts quickly to an impulse, but only after a certain limit of force has been exceeded; on the other hand, he calls it “a viscous fluid” when it reacts to an infinitely small impulse, although infinitely slowly. “When this con-
