Humphry Davy’s theory of Earth’s interior

Humphry Davy’s theory of Earth’s interior becomes the expedition’s counterargument to the predicted inferno beneath the crust: it allows Harry and Professor Hardwigg to interpret a cool underground environment, and eventually a real subterranean sea, as evidence against a simple law of steadily increasing heat. Its force in the narrative comes from a contest between inherited scientific expectation and what the travellers measure and see, not from an abstract exposition alone.

The theory against the expected central heat

At the outset, Harry objects that established scientific teaching makes a journey into the Earth impossible. He invokes the familiar estimate that temperature rises by one degree for every seventy feet of descent: carried inward, he argues, the rule implies that even gold, platinum, and the hardest rocks would melt. Professor Hardwigg refuses to let theory settle the question in advance. Their disagreement establishes the central problem the theory will later answer: whether the travellers must treat received calculations as decisive, or allow observations underground to revise them.

The narrative does not make Harry’s objection foolish. As the party passes through changing rock formations, he reads their geological sequence as evidence about the direction and depth of their journey. In a transition formation associated with Silurian stone, he connects the strata to sediment laid down beneath ancient waters; later, he sees fossil remains in the marbles and recognizes a more developed order of ancient animal life than the trilobites they had encountered earlier. Such evidence lends material weight to his geological reasoning, even as Professor Hardwigg remains determined to continue.

Measurements and competing explanations

The contradiction sharpens when Harry calculates that the expedition has descended about sixteen leagues, or fifty miles, already the extreme limit he understands science to allow for the thickness of the Earth’s crust. By the familiar rule of increasing heat, he expects a temperature of roughly fifteen hundred degrees Réaumur. Instead, the thermometer reads only 27.6 degrees. Professor Hardwigg treats the disparity—1,474.4 degrees—as evidence that the proportional increase of temperature is an exploded error, and credits Humphry Davy for having been right.

Harry does not simply concede the theory. He continues to hold to the expected increase in heat, while proposing a different explanation for the low reading: the chimney of an extinct volcano may be covered with lava that resists heat and conducts it poorly, preventing the deeper heat from passing through its walls. The hot-water jet, in his view, supports this account. Rather than prolong the dispute with another abstract argument, he accepts Professor Hardwigg’s calculations provisionally and draws a further conclusion from them: at their location under Iceland, the terrestrial depth should be about 1,583 leagues. The episode distinguishes the measured fact from the explanations the men build upon it; both are forced to reason from the same unexpected temperature.

The theory also appears in Professor Hardwigg’s reading of volcanic signs. He explains that, as an eruption approaches, steam vents grow more active, then disappear during the eruption as elastic fluids seek refuge inside the crater rather than escaping through fissures. If the vapour remains in its normal state and there is no shift to heavy atmospheric pressure and dead calm, he concludes that an immediate eruption is not to be feared. His confidence in this rule shows how the expedition uses scientific interpretation not only to explain the underground world but to judge its immediate dangers.

The sea as a test of belief

When the travellers reach the underground sea, Harry acknowledges how far observation has unsettled his expectations. He had considered an interior ocean with tides, winds, and storms almost unimaginable; once the central-heat theory is set aside, however, he can offer no physical reason that such a sea must be impossible. Professor Hardwigg asks whether this validates Sir Humphry Davy’s system, and Harry grants that it does. That concession does not make him accept every marvel without reservation: he still thinks the interior countries are more likely uninhabited, while allowing that the sea might shelter unknown fish.

The sequence makes the theory’s persuasive power cumulative. A thermometer’s modest reading first challenges the predicted heat; the underground sea then turns that numerical discrepancy into a world that the old expectation could not readily accommodate. Yet the novel preserves a distinction between observing the sea and deciding what general theory explains it. Harry’s concession follows what the travellers have seen, while his continuing qualification shows that evidence changes his position without erasing his habits of doubt.

Scientific knowledge and the deep past

Elsewhere, the narrative places the expedition’s underground observations beside contemporary controversy over human antiquity. It recounts the 1863 discovery of a human jawbone buried fourteen feet below the surface at Moulin-Quignon, near Abbeville, alongside stone hatchets and carved flints. The bone’s authenticity won influential supporters, including Professor Hardwigg, while Elie de Beaumont disputed the age of its geological setting and rejected the idea that humans had lived alongside Quaternary animals. Further jawbones from caves in France, Switzerland, and Belgium strengthened the case for human presence in the Quaternary; worked fossil bones from still older deposits prompted some scholars to argue for human activity in the Pliocene.

This account does not directly prove Humphry Davy’s theory. Instead, it establishes the period’s unsettled intellectual landscape, in which discoveries and their interpretation could move the boundaries of accepted time. The narrative’s geological fossils perform a similar function within the journey: the travellers read successive formations and animal remains as evidence of a changing history beneath their feet. In both contexts, material traces invite conclusions that reach beyond what can be seen immediately, and those conclusions remain subject to disagreement.

Humphry Davy’s theory therefore functions as a practical alternative to a fixed expectation of central heat and as a measure of the expedition’s changing relation to scientific authority. Professor Hardwigg welcomes observations that overturn the temperature rule; Harry resists abandoning his explanation until the evidence becomes difficult to contain within it. Their debate gives the subterranean discoveries intellectual stakes: the journey does not merely reveal wonders, but repeatedly asks how a claim should change when the world observed no longer fits the theory that predicted it.