X - that there is considerable skepticism - shared by me! -...

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X-Rays: A doctor can have a patient stand in front of a source of X-rays and record, on photographic emulsion, what comes through  and what is shadowed by the bones and other material in the path. It is not possible to do anything equivalent for the Earth,  although the concept of `shadow zones' returns in a slightly different guise in seismological studies of the Earth. The Aura: There are ways of studying the Earth which have no immediate medical analogue, one of which is to infer something about  the deep interior from the presence, strength, and behaviour of the Earth's magnetic field.  Although the analogy is imperfect,  this might best be seen as comparable to the study of a person's aura. (The analogy is weak in various ways, one of which is 
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Unformatted text preview: that there is considerable skepticism - shared by me! - about the reality of personal auras.) As we will learn, the Earth's magnetic field has an importance which far transcends the merely diagnostic, however: the magnetic field, and especially the way in which it periodically changes, turns out to provide a critical link in developing our understanding of the way in which Continental Drift works. (See pages 400-404 of your text.) Ultrasound: The very best diagnostic tool for geophysical studies turns out to be one which is analogous to the use of ultrasound on the human body. But this is so important that it deserves a section on its own....
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