Central-fire theory

Central-fire theory is the proposition that intense heat, ultimately associated with a burning or molten interior, exists beneath the Earth’s crust. In the novel it is not a settled scientific backdrop but a contested explanation repeatedly tested against the expedition’s measurements, geology, and living observations. The argument shapes the decision to descend, then returns underground as Harry and Professor Hardwigg interpret evidence that refuses to fit comfortably within either man’s expectations.

The theory as danger and intellectual wager

Before the journey, Harry argues that descent is impossible according to scientific teaching. He points to the accepted estimate that temperature rises by one degree for every seventy feet underground: carried down to the planet’s centre, that gradient implies that the globe’s materials, including metals and hard rocks, would be incandescent or molten. His question is practical as well as theoretical: what could happen to the expedition in such heat?

Professor Hardwigg rejects the inference rather than accepting its danger. He insists that no one knows the Earth’s actual interior, and argues that if the supposed heat existed in the form Harry describes, the upper crust would be shattered and the world destroyed. For him, older theories do not deserve unquestioned authority when modern experiments unsettle them. Their disagreement therefore concerns how to reason from incomplete knowledge: Harry treats established temperature estimates as a warning, while Hardwigg treats uncertainty itself as grounds for investigation.

Hardwigg’s answer turns theory into a wager on observation. He dismisses the dangers Harry multiplies and holds that the only way to learn is to go and see, following the precedent of Arne Saknussemm. The proposition that the interior may be traversable is inseparable from the challenge to the central-fire model: if received accounts of underground heat are unreliable, the journey may be possible; if they are right, the descent could be fatal.

Iceland as apparent proof

On the approach to Sneffels, the landscape appears to support Harry’s original position. He reads Iceland’s geology as the record of sustained subterranean fires: dark basalt lies across the plain in heavy, cooling, six-sided prisms; flattened cones mark former volcanic vents; and lava, cinders, and pumice testify to eruptions. From this sequence he concludes that the planet’s central mass must remain liquid and white-hot. If that conclusion is sound, penetrating the Earth is an absurd undertaking.

The thought reassures Harry precisely because it seems to end the expedition before it begins. He treats the volcanic ground as a demonstration of central fire and, with that argument in mind, goes forward toward the mountain as if advancing to an assault. The irony is structural: evidence that appears to confirm the danger does not turn the travelers back. Instead, the disputed theory becomes part of the intellectual pressure carried into the descent.

The explanation of volcanic activity also appears in Hardwigg’s practical account of eruption. He says that steam vents become more active as an eruption approaches, then disappear during the eruption when elastic fluids seek refuge inside the crater rather than escaping through fissures. If their activity remains normal and the atmospheric conditions do not shift to heavy pressure and dead calm, he judges an immediate eruption unlikely. Here the theory is used not as a general account of the whole Earth, but as a framework for reading observable signs and assessing a specific danger.

Descent, strata, and competing explanations

Once underground, Hardwigg initially interprets the volcanic strata against the central-fire theory. He tells Harry that the arrangement of the layers confirms Humphry Davy’s account of the Earth’s interior: they are still in primordial soil, where metals became inflamed through contact with air and water. On this reading, the old theory of a central fire is exploded. Hardwigg’s confidence in the journey thus rests not on a single fixed explanation, but on his willingness to replace one model with another as he reads the evidence.

Harry, meanwhile, has little appetite for geological classification during the descent. At a halt Hardwigg lectures on the strata; Harry’s attention is fixed on the danger and darkness, and his silence is taken as consent. The difference in attention matters: the Professor converts layers into a running scientific record, while his nephew experiences the same passage as a physical ordeal whose outcome the theory may determine.

A later calculation makes the dispute concrete. Harry observes that the thermometer reads only about fifteen degrees Réaumur, an increase of nine degrees from their departure. He applies an estimate of one degree per hundred feet, while allowing for local variation: near Yakoutsk the increase has been observed at one degree per thirty-six feet, and near an extinct volcano at one degree per twenty-five feet. Using the most favorable of these figures for his calculation, he arrives at a depth of 1,125 feet from the temperature increase alone.

That calculation does not match their position. From his observations of their route, Harry estimates that they are at least 10,000 feet below sea level; the Professor’s reckoning confirms that they are already 6,000 feet deeper than anyone had previously reached, beyond the known depths of the mines at Kitzbuhel and in Wurttemberg. Yet the thermometer shows only fifteen degrees where the expected temperature should be eighty-one. The disparity is not a minor curiosity: it places direct measurement in conflict with the projected increase in subterranean heat and makes the instruments central to the debate.

Harry’s skepticism later returns as an argument about the direction and depth of their route. He asks what proves that the lava does not extend to the Earth’s centre and suggests that they have not yet descended far enough below sea level for the temperature to settle the question. When Hardwigg asks what the thermometer says, Harry reports a reading of 27.6 degrees. The Professor calculates that this is 1,474.4 degrees below the figure expected by the temperature rule and treats the discrepancy as proof that proportional increase is an exploded error. In the same breath, he celebrates Humphry Davy’s theory and his own decision to trust it. The measurement shakes one model, but Hardwigg’s response also shows how quickly an anomaly can be recruited to defend another.

Geological time and the return of subterranean fire

The explorers’ route supplies evidence of deep geological time as well as temperature. In a transition zone, Harry identifies Silurian rocks, describing schists and calcareous formations as deposits left by waters that once covered the Earth. Fossil plants, including fucus and lycopods, appear on the gallery walls. When Harry presents a nearly complete shell, Hardwigg identifies it as a trilobite, an extinct crustacean. Farther on, marbles bear the marks of more developed ancient life, including remains of fish associated with the Devonian seas. For Harry, the changing fossils make the passage through the strata a visible ascent in the scale of animal life; Hardwigg continues onward rather than accepting these signs as a reason to turn back.

These observations place the temperature dispute inside a larger question of how to read the underground world. Strata, fossils, and living creatures are not merely illustrations of a theory: they are evidence encountered in sequence, classified by travelers whose interpretations can diverge. The route makes geological history palpable, but it does not automatically settle the arguments through which that history is explained.

The narrative later makes a forceful assertion in the other direction: despite Hardwigg’s learned theories, a fierce fire burns within the Earth, its action reaching the crust. In this account, primordial plants live without the sun’s vivifying rays, while their roots draw strength from the heat of the early Earth. Vast vegetation contributes to the origin of coal; the crust yields above a seething mass, fissures open, and upper layers repeatedly fall in. Central fire is thus no longer only the frightening extrapolation Harry raised before the journey or the obsolete theory Hardwigg rejected below ground. It becomes an explanatory image for the formation of the ancient world through which they travel.

The expedition’s striking encounter with a living fish adds another kind of evidence. Its flat head, rounded body, bony scales, toothless mouth, and unusually developed pectoral fins initially invite Harry to compare it with a sturgeon. Hardwigg’s careful examination rejects that identification: the fish belongs to a family otherwise known only from fossil remains in Devonian strata. Like the temperature readings, the encounter makes knowledge depend on the relation between an observed object and the categories used to explain it; what seems familiar at first proves to belong to an extinct world.

Significance and limits of the argument

The novel gives central-fire theory no simple status as a doctrine that the explorers either prove or disprove. At the outset it is a threat calculated from a familiar geothermal rule; in Iceland, volcanic rocks seem to confirm it; during the descent, Hardwigg invokes Davy against it; and the thermometer supplies numbers that undermine expected heat without erasing the question of what lies deeper. Later descriptions of primordial fire give the theory renewed explanatory force. These shifts make the disagreement a record of interpretation under pressure, not a clean progression from error to certainty.

The contrast between theory and direct observation is especially sharp because the travelers rely on instruments and material traces as well as authority. Hardwigg invokes scientific judgment and Davy; Harry produces calculations from thermometer readings, points to fossils, and questions whether the route has gone far enough to support a conclusion. But neither side possesses a view of the Earth’s centre independent of inference. Measurements have to be related to depth, and fossils to geological history; anomalous results can unsettle a claim while still leaving room for competing explanations.

The same tension returns during the crisis near the end of the journey. Faced with boiling water, excessive heat, altered granite, and an approaching electrical disturbance, Harry examines the compass and finds its needle darting from pole to pole. That reversal does not itself identify the source of the heat, but it renews the encounter with a material Earth that resists ordinary expectations. Central-fire theory therefore matters both as a scientific proposition and as a recurring narrative test: whenever the explorers believe that inherited laws have settled what is possible, the world below supplies another observation that forces them to reconsider how much they know.