Slide 1

In the 19th century, scientists observed that the densities of rocks near the Earth's surface are not high enough to explain the Earth's rotation and gravity. In 1896, a German seismologist named Emil Weichert proposed a solution: a two-layered model of the Earth: a dense, iron core in the center of the Earth and a layer of rocks on the surface. To verify his theory, he set up the first seismic station near Gottingen, Germany in 1902, complete with a seismograph and an earthquake-simulator illustrated by Figure 1.

Figure 1. Taken from https://interestingengineering.com/this-4-ton-steel-ball-causes-artificial-earthquakes-to-help-us-understand-the-earths-crust

This earthquake simulator, an invention of his student Ludger Mintrop, consists of a 4-ton steel ball that could be dropped 14 feet to create seismic waves that would allow Weichert and Mintrop to investigate the ground immediately beneath their station. Today, this seismic station still stands, and Wiechert's machines are still running, having been in continuous use since 1903. The technique he and Mintrop developed for ground investigations is still used as well, although today we use explosives.