All the talk of barometers brings me back to this.
The pressure wave from the Krakatoa explosion spread outwards in all directions, like the rings formed when you throw a stone into a pond, around the globe, reached its antipode in Honduras and returned to the epicenter.
In geography, the antipode of any spot on Earth is the point on Earth's surface diametrically opposite to it. A pair of points antipodal to each other are situated such that a straight line connecting the two would pass through Earth's center. Antipodal points are as far away from each other as possible.
As I said the wave was recorded, using barographs, by independent observers at various points on the earth. When the data was collected and collated it was seen that the wave converged at one point on the earth, Honduras, which was the antipode of the eruption, the epicentre.
One point on the earth. Not at several points around a continuous gradient that arcs up into the dome. Like the sides of your bath.
I can’nt explain or imagine how this could happen on any shape other than a sphere, can you?
Or, if the earth is not a globe, as Honduras is the farthest point from the epicentre the wave reached before it started on its return, Honduras must lie on a place where the continuous gradient that arcs up into the dome is.
This took place in 1883, as DaveH maintains, barographs have been around for some time.
You do believe that Krakatoa erupted and that the magnitude of the eruption created a pressure wave that was mapped as it travelled across the earth?