On August 27, 1883 the volcanic island of Krakato suffered a cataclysmic eruption. The sound was loud. No, it was
SUPER LOUD!!! It ruptured eardrums 40 miles away. It could be heard by actual human ears at distances of thousands of miles away, the most distant audible sound reported by an observer on an Indian Ocean island 3,000 miles away. Point is, this sound was YUUUGE!
Sound is an air pressure wave. Even as the sound level fell below the threshold of human hearing, the sound wave continued to propogate away from the source. Hours after the blast, the barometers installed at weather stations around the world saw the spike in air pressure as the wave reached their location. (This is why I used the term "actual human ears" above, to distinguish that observation from the subsequent instrument-based observations). Six hours and 47 minutes after the Krakatoa explosion, a spike of air pressure was detected in Calcutta. By 8 hours, the pulse reached Mauritius in the west and Melbourne and Sydney in the east. By 12 hours, St. Petersburg noticed the pulse, followed by Vienna, Rome, Paris, Berlin, and Munich. By 18 hours the pulse had reached New York, Washington DC, and Toronto. Amazingly, for as many as 5 days after the explosion, weather stations in 50 cities around the globe observed this unprecedented spike in pressure re-occuring like clockwork, approximately every 34 hours. That is roughly how long it takes sound to travel around the entire planet.
There was
great interest in these pressure observations in subsequent years. In those days any subject of the British Empire who fancied himself to have a certain level of sophistication, especially if one was posted abroad in the Empire, had his own suite of weather instruments. When it became known that this sound wave was observable in barometric recordings, everybody checked theirs. And sure enough, from all over the world came reports of amateur observers who had captured one or more pulses.
Now, the shape of world in 1883 was thought to be round. These pressure observations from all over the Empire, caused by an event occurring at a known time, provided an opportunity to prove otherwise, were the world some other shape. However, the timing of the pressure wave's arrival at each barometer corresponded to what one would expect for a globe.