James Stewart

Now thankfully there's one way we can get particularly insightful information, and that is from earthquakes.

By the 19th century, increased interest in seismic activity had prompted the creation of a device to measure them scientifically.

Yeah, this was the modern seismograph, and it looks something like this.

For the first time, the analysis of seismic events weren't based on eyewitness observation and hearsay, but on quantifiable data, which we love here on the channel.

But while the device itself was unquestionably a scientific breakthrough, the real magic happened when the invention spread across the globe.

As seismographic networks expanded, scientists found they could detect earthquakes happening as far away as the other side of the planet.

In 1897, the so-called Great Earthquake erupted at Shillong Plateau in India.

Shocks were felt in Calcutta and even as far away as Burma.

But seismographs detected this earthquake much, much further away, in Europe, where the seismograms they created were pored over by British geologist Richard Dixon Oldham.

Leader of the Geological Survey of India, Oldham noticed that the seismogram showed three distinct wave shapes.

In his papers, he compared the data found from this and several other earthquakes and found that they too showed these same three waves.

The reason he could see them was, being far from the epicentre, differing speeds of the waves meant they arrived one at a time.

The first arrivals were later known as primary waves, pressure waves, or P waves, which are longitudinal.

They oscillate in the same direction as they're traveling in.

These were then followed by secondary, shear, or S waves, which are transverse.

Their oscillations are perpendicular to their direction of travel.

Finally, surface waves ripple through the Earth's crust, and it is these that tend to cause the most destruction.

It had already been established by this point that longitudinal waves can pass through both solids and liquids, but transverse waves can only pass through solids, and you can probably see where this is going.

Since the seismic waves had travelled through the inside of Earth to reach the seismograms, the resulting seismographs could yield information about what they passed through, namely, the structure of Earth itself.

This realisation would allow centuries of speculation to finally be put to rest.