Free Astronomy Magazine September-October 2019

changes in atmospheric pressure at the first light of day. Similar observations were made in Jena and Potsdam, in Germany, but also in Washington, where they appeared less intense than the Euro- pean ones. The few Asian recordings were far more marked. Particularly interesting were the Potsdam measurements, which revealed both direct waves, recorded on the morning of June 30, and return waves, recorded on the morning of July 1 st . Similar measurements were made by observation stations in Cambridge and Petersfield, England, thanks to which it was possible to establish that the pressure wave had passed at a speed of just over 300 meters per second, absolutely comparable to that pro- duced in 1883 by the Krakatoa volcano explosion. Further east in Kansk, Siberia (about 500 km north of Mongolia), an observing station recorded violent seismographic and barographic oscillations and in- formed the Central Seismic Commission of St. Pe- tersburg. The Commission, however, considered the report to be senseless since Siberia was not consid- ered a seismic zone. Recordings of that same type performed by the Irkutsk Magnetic and Meteoro- logical Observatory, another observational station