I’m glad you brought up the question of our investigations into the makeup of the earth’s interior.  In fact, since this is the topic of your reading assignment for next time, let me spend these last few minutes of class talking about that.
There were several important discoveries in the early part of the century to help geologists develop a more accurate picture of （the earth’s interior）.（ The first key discovery） had to do with seismic waves. Remember, they are the vibrations caused by earthquakes. Well, scientists found that they travel thousands of miles through the earth’s interior. This finding enabled geologists to study the inner parts of the earth.
You see, these studies revealed that these vibrations were of two types: compression or P waves and shear or S waves. And researchers found that P waves travel through both liquids and solids while S waves travel only （through solid matter）. In 1906, a British geologist discovered that P waves slowed down at a certain depth but kept traveling deeper. On the other hand, S waves （either disappeared or were reflected back）.  So he concluded that the depth marked the boundary between a solid mantle and the liquid core. Three years later, （another boundary） was discovered, that between the mantle and the earth’s crust.
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