Many things happen in September. It’s a time to go back to school, to look forward to a change in the weather, to do fall gardening, to plan Halloween costumes, and it’s a time to remember Inge Lehmann’s groundbreaking paper that concludes that the Earth has a solid inner core.
The year was 1936. Thoughts of what might be under the Earth’s crust were present in the public cultural imagination, thanks in part to Jules Verne’s classic and other works of late 19th century subterranean fiction (Will Harben’s Land of the Changing Sun (1894), Gabriel Tarde’s Underground Man (1896), Charles Beale’s The Secret of the Earth from 1899, to name a few).The late 19th century and turn of the 20th was also an important time in the history of seismology. According to Ari Ben-Menahem, seismology “aims simultaneously to obtain the infrastructure of the Earth's interior with the aid of seismic wave phenomena, and to study the nature of earthquake sources with the ultimate goal of mitigating and eventually controlling the phenomenon.” In other words, seismologists concern themselves with the study of earthquakes and what is under the Earth’s crust. The first seismometer (an instrument with a pendulum or spring that can record earthquakes, volcanic eruptions, and explosions) was designed by James Forbes in 1841. In 1894, the term seismograph appeared, which is now often interchangeable with seismometer, to describe the physical recording of ground displacements. By the early 20th century, scientists were able to tell through seismographs that all was not solid under the Earth’s crust.