What is the important discovery about seismic waves mentioned by the instructor?

There were several important discoveries in the early part of last century that helped geologists develop a more accurate picture of the earth’s interior.(15 - 1)The first key discovery had to do with seismic waves. Remember they are the vibrations caused by earthquakes. Well, (15-2)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, the studies revealed that these vibrations were of two types: compression, or P waves, and shear, or S waves.(17)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 a liquid core. Three years later, another boundary was discovered that was between the mantle and the earth’s crust. There is still a lot to be learned about the earth. For instance, geologists know that the core is hot. Evidence of this is the molten lava that flows out of volcanoes. But we are still not sure what the source of the heat is.