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Interior of the Earth—Illustration from Norway—Solids and Liquids—Rigidity of the Interior of the Earth—Earthquakes, how caused—Their Testimony as to the Rigidity of the Earth—Delicate Instrument for Measuring Earthquake Tremors—The Seismometer—Professor Milne’s Work in the Isle of Wight—Different Earthquake Groups—Precursors and Echoes—Vibrations transmitted through the Earth’s Centre—Earthquakes in England—Other Evidence of the Earth’s Rigidity—Krakatoa, August 27th, 1883—The Sounds from Krakatoa—The Diverging Waves—The Krakatoa Dust—The Hurricane Overhead—Strange Signs in the Heavens—The Blood-red Skies. In this chapter we shall learn what we can as to the physical condition of the interior of our earth so far as it may be reasonably inferred from the facts of observation. We have already explained in the last chapter that a very high temperature must be found at the depth of even a small fraction of the earth’s radius, and we have pointed out that the excessively high pressure characteristic of the earth’s interior must be borne in mind in any consideration as to the condition of the matter there found. Let us take, for instance, that primary question in terrestrial physics, as to whether the interior of the earth is liquid or solid. If we were to judge merely from the temperatures reasonably believed to exist at a depth of some twenty miles, and if we might overlook the question of pressure, we should certainly say that the earth’s interior must be in a fluid state. It seems at least certain that the temperatures to be found at depths of two score miles, and still more at greater depths, must be so high that the most refractory solids, whether metals or minerals, would at once yield if we could subject them to such temperatures in our laboratories. At such temperatures every metal would become fluid, even if it were not transformed into a cloud of vapour. But none of our laboratory experiments can tell us whether, under the pressure of thousands of tons on the square inch, the application of any heat whatever would be adequate to transform solids into liquids. It may indeed be reasonably doubted whether the terms solids and liquids are applicable, in the sense in which we understand them, to the materials forming the interior of the earth.
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