The reason for this is that the velocity for P-waves and S-waves is governed by both the different properties in the material which they travel through and the different mathematical relationships they share in each case. The three properties are: incompressibility ($k$ ), density ($p$ ) and rigidity ($u$ ). P-wave velocity is equal to ${\sqrt {(k+{\tfrac {4}{3}}u)/p}}$ whereas S-wave velocity is equal to ${\sqrt {(u/p)}}$ and so S-wave velocity is entirely dependent on the rigidity of the material it travels through. Liquids, however, have zero rigidity, hence always making the S-wave velocity overall zero and as such S-waves lose all velocity when travelling through a liquid. P-waves, however, are only partially dependent on rigidity and as such still maintain some velocity (if greatly reduced) when travelling through a liquid. Analysis of the seismology of various recorded earthquakes and their shadow zones led geologist Richard Oldham to deduce in 1906 the liquid nature of the Earth's outer core.