Richard Oldham. Portrait of Richard Dixon Oldham (1858-1936), the Irish geologist and seismologist who discovered the Earth's core. Oldham spent much of his professional life in India. In 1897 he dis- tinguished between P (primary or pressure) and S (secondary or shear) seismic waves. He also desc- ribed many other characteristics of earthquake activity. In 1906 he discovered that P waves reached points on the opposite side of the Earth from the earthquake epicentre later than expected. He correctly deduced that this was because the Earth had a relatively dense core through which the P waves travelled more slowly than they did through the less dense mantle.