The Seismograph is like the Spectroscope in Importance

Just as the spectroscope opened up a new astronomy by enabling the astronomer to determine some of the constituents of which distant stars are composed, so the seismograph, recording the unfelt motion of distant earthquakes, enables us to see into the earth and determine its nature with as great a certainty, up to a certain point, as if we could drive a tunnel through it and take samples of the matter passed through.

Richard Dixon Oldham, The Constitution of the Interior of the Earth, as Revealed by Earthquakes (1906)

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