Internal Structure of the Earth
From the study of magnetic compass variations, Halley by 1683 had reached the quite original conclusion that the Earth possessed four magnetic poles (11). He described in the Philosophical transactions of that year how two of these poles were located in the “Southern ocean” and the two northern ones were in the Bering Strait and Spitzbergen (12). However he could not account for the existence of multiple poles, nor their gradual displacement with time, acknowledging that the latter depended on “secrets as yet utterly unknown to Mankind” (13).
Various attempts had been made to account for the gradual motion of the lines of magnetic declination by a few minutes of arc each year, as this had great relevance to navigation.(14) Descartes for example had suggested that such motion was due to physical accretion and movement of iron ore deposits (15), but Halley realized during his 1676 voyage to St Helena that no theory depending on surface deposits of iron could explain the compass variations. Henry Bond claimed to be able to predict secular changes in terrestrial magnetism, and in 1674 the King set up a committee to investigate this claim (16). Robert Hooke, who was a member of the committee, proposed in 1674 that the magnetic poles were moving in a circular path at 10° from the geographic poles, possibly rotating once in 370 years (17).
Halley had gathered much data on the subject from his voyage to St Helena, as he also had from the unpublished manuscripts of Peter Perkins. Perkins had been researching this topic and addressed the Royal Society on the subject in 1680 some months before his death. Halley purchased Perkins’s papers on the variation of geomagnetism immediately following the latter’s demise (18).While it is true that Halley never acknowledged this, it is equally true that nothing has substantiated Flamsteed’s allegation that components of Halley’s theory derived from Perkins. Perkins was developing a theory of his own to account for the variation, as was indicated in his address to the Royal Society, but it is hard to discern any resemblance to that developed by Halley (19). The first part of Halley’s 1692 essay commented on the inadequacy of existing theories, concluding that something deep below Earth’s surface must be causing the phenomenon.
Halley had been much involved with the production of the Principia, and it now seemed to provide him with a key. Its estimate of the Earth/Moon mass ratio suggested to him that the Earth was hollow. How else could that ratio be explained? The germ of the idea may have dawned upon him while reading Burnet’s Sacred theory of the Earth which had appeared (in Latin) in 1681. This assigned hollow cavities to the Earth, catacombs and subterranean grottoes, but did so in a traditional mode in accord with classic myth and lore (20). The tenor of Burnet’s vision was, as Schaffer has observed (21), in stark contrast to that of Halley.
With this assumption, Halley could gain a physical explanation for the existence of his four magnetic poles, and also their motions. Within the Earth, concentric to its hollow shell and rotating coaxially, he discerned another sphere, whose existence had hitherto been unsuspected. Possibly there were further spheres hidden within that one. Both spheres had magnetic poles embedded within them at a distance from their common axis of rotation. A very gradual differential rotation between the two spheres accounted for the drift of the magnetic poles with time. One pair of magnetic poles was stationary, being embedded in the outer shell, while the other pair drifted westwards, because the inner sphere was revolving at a slower rate. Of the two northern poles, Halley opined that the pole due north of Land’s End was the moving one, while the other lying on a meridian through California, at some 15° from the pole, was static. Of the two southern poles, that south of America was the moving one.
Halley’s Earth was composed of an outer shell 500 miles thick, with an air gap of the same distance between it and the inner sphere. To the objection that the latter might collide with the outer shell, and thereby damage it, he explained that it was held at the centre by the force of gravity. Halley was confident his readers would perceive the necessity of this: “should these globes be adjusted once to the same common centre, the Gravity of the parts of the Concave would press towards the centre of the inner ball ... it follows that the Nucleus being once fixt in the common centre, must always here remain (22). Halley pointed out that “the Ring environing the Globe of Saturn”, which remained coaxial to the planet, was held there by gravity. (No-one then knew that Saturn’s rings were rotating. The Principia had not discussed the matter.) By analogy, could not gravity also hold a globe concentric inside the hollow Earth?
Happily, Halley’s argument was in accord with “Almighty Wisdom”, which would not have arranged all the matter of Earth “barely to support its surface”. Rather, the matter had been distributed “to yield as great a Surface for the use of living Creatures as can consist with the conveniency and security of the whole”. Halley acknowledged that there might be objections to his new theory: for example, might not the oceans leak away if an earthquake opened up a crack beneath them, leading to flooding of the lower regions? His answer was that within the ground there existed “saline and Vitriolick Particles as may contribute to petrefaction”. Water flowing down would soon find its path blocked by these petrifying particles.
“I have adventured to make these Subterranean orbs capable of being inhabited”, Halley added, in words for which science fiction writers of futurity would be grateful (23). His reason for this was, that all nature teemed with life. The argument was teleological. His under-world would be occupied, as the planets too were doubtless inhabited, and so some illumination was therefore needed: “The concave arches may in several places shine with such a substance as invests the surface of the Sun.” Halley conceded that he was here using a “final cause” in asserting that the luminosity of the upper atmospheres of his under-world was provided for its denizens.
It has been argued by David Kubrin that Halley took over Robert Hooke’s view of the Earth as “a series of concentric shells, one of which contained the magnetic poles”, and that Halley’s model of internal terrestrial structure was “rather similar to Hooke’s (24). David Oldroyd has likewise claimed that “... Hooke’s theory was thought sufficiently adequate to furnish a model for the structure of the Earth by Halley in 1692, in his explanation of the Earth’s magnetic poles (25). There are however objections to this view.
Hooke’s model of the Earth’s interior was layered like an onion, and hardly resembled that developed by Halley (26). There is indeed no single feature which the two views shared in common. Hooke’s model lacked any air-gaps between its strata, or subterranean spheres, or principle of differential rotation, or multiple magnetic poles; nor had it any mechanism for a rotation of magnetic poles around the geographic poles. Only, in Hooke’s view, in the Earth’s pristine condition may it once have had spherical concentric shells of matter, but earthquakes had long since disrupted that ideal scheme. Hooke did not locate the Earth’s magnetic power in a specific shell as Kubrin claimed, but rather left it diffusely spread through the “magnetical core or Magnetical Globe of the Earth (27). These views appeared in Hooke’s posthumously published essay “On earthquakes”. In 1674 Hooke gave his opinion to the Royal Society that the Earth’s magnetic poles were rotating around the Earth’s axis, but this was accompanied by no suggestion as to how such an effect could be produced: he never theorized, as did Halley, as to what might produce a periodic rotation of the magnetic poles. For Hooke, earthquakes were an instrument for the transformative processes that he envisaged through the Earth’s geological history, whereas on Halley’s model they presented a difficulty which required accounting for. It therefore appears that not one of the highly distinctive characteristics of Halley’s model came from Hooke. Halley’s theory was original, and derived not from the lectures of Hooke but rather from the error in lunar mass of Newton’s Principia.
Internal Structure of the EarthFrom the study of magnetic compass variations, Halley by 1683 had reached the quite original conclusion that the Earth possessed four magnetic poles (11). He described in the Philosophical transactions of that year how two of these poles were located in the “Southern ocean” and the two northern ones were in the Bering Strait and Spitzbergen (12). However he could not account for the existence of multiple poles, nor their gradual displacement with time, acknowledging that the latter depended on “secrets as yet utterly unknown to Mankind” (13).Various attempts had been made to account for the gradual motion of the lines of magnetic declination by a few minutes of arc each year, as this had great relevance to navigation.(14) Descartes for example had suggested that such motion was due to physical accretion and movement of iron ore deposits (15), but Halley realized during his 1676 voyage to St Helena that no theory depending on surface deposits of iron could explain the compass variations. Henry Bond claimed to be able to predict secular changes in terrestrial magnetism, and in 1674 the King set up a committee to investigate this claim (16). Robert Hooke, who was a member of the committee, proposed in 1674 that the magnetic poles were moving in a circular path at 10° from the geographic poles, possibly rotating once in 370 years (17).Halley had gathered much data on the subject from his voyage to St Helena, as he also had from the unpublished manuscripts of Peter Perkins. Perkins had been researching this topic and addressed the Royal Society on the subject in 1680 some months before his death. Halley purchased Perkins’s papers on the variation of geomagnetism immediately following the latter’s demise (18).While it is true that Halley never acknowledged this, it is equally true that nothing has substantiated Flamsteed’s allegation that components of Halley’s theory derived from Perkins. Perkins was developing a theory of his own to account for the variation, as was indicated in his address to the Royal Society, but it is hard to discern any resemblance to that developed by Halley (19). The first part of Halley’s 1692 essay commented on the inadequacy of existing theories, concluding that something deep below Earth’s surface must be causing the phenomenon.Halley had been much involved with the production of the Principia, and it now seemed to provide him with a key. Its estimate of the Earth/Moon mass ratio suggested to him that the Earth was hollow. How else could that ratio be explained? The germ of the idea may have dawned upon him while reading Burnet’s Sacred theory of the Earth which had appeared (in Latin) in 1681. This assigned hollow cavities to the Earth, catacombs and subterranean grottoes, but did so in a traditional mode in accord with classic myth and lore (20). The tenor of Burnet’s vision was, as Schaffer has observed (21), in stark contrast to that of Halley.With this assumption, Halley could gain a physical explanation for the existence of his four magnetic poles, and also their motions. Within the Earth, concentric to its hollow shell and rotating coaxially, he discerned another sphere, whose existence had hitherto been unsuspected. Possibly there were further spheres hidden within that one. Both spheres had magnetic poles embedded within them at a distance from their common axis of rotation. A very gradual differential rotation between the two spheres accounted for the drift of the magnetic poles with time. One pair of magnetic poles was stationary, being embedded in the outer shell, while the other pair drifted westwards, because the inner sphere was revolving at a slower rate. Of the two northern poles, Halley opined that the pole due north of Land’s End was the moving one, while the other lying on a meridian through California, at some 15° from the pole, was static. Of the two southern poles, that south of America was the moving one.Halley’s Earth was composed of an outer shell 500 miles thick, with an air gap of the same distance between it and the inner sphere. To the objection that the latter might collide with the outer shell, and thereby damage it, he explained that it was held at the centre by the force of gravity. Halley was confident his readers would perceive the necessity of this: “should these globes be adjusted once to the same common centre, the Gravity of the parts of the Concave would press towards the centre of the inner ball ... it follows that the Nucleus being once fixt in the common centre, must always here remain (22). Halley pointed out that “the Ring environing the Globe of Saturn”, which remained coaxial to the planet, was held there by gravity. (No-one then knew that Saturn’s rings were rotating. The Principia had not discussed the matter.) By analogy, could not gravity also hold a globe concentric inside the hollow Earth?Happily, Halley’s argument was in accord with “Almighty Wisdom”, which would not have arranged all the matter of Earth “barely to support its surface”. Rather, the matter had been distributed “to yield as great a Surface for the use of living Creatures as can consist with the conveniency and security of the whole”. Halley acknowledged that there might be objections to his new theory: for example, might not the oceans leak away if an earthquake opened up a crack beneath them, leading to flooding of the lower regions? His answer was that within the ground there existed “saline and Vitriolick Particles as may contribute to petrefaction”. Water flowing down would soon find its path blocked by these petrifying particles.“I have adventured to make these Subterranean orbs capable of being inhabited”, Halley added, in words for which science fiction writers of futurity would be grateful (23). His reason for this was, that all nature teemed with life. The argument was teleological. His under-world would be occupied, as the planets too were doubtless inhabited, and so some illumination was therefore needed: “The concave arches may in several places shine with such a substance as invests the surface of the Sun.” Halley conceded that he was here using a “final cause” in asserting that the luminosity of the upper atmospheres of his under-world was provided for its denizens.
It has been argued by David Kubrin that Halley took over Robert Hooke’s view of the Earth as “a series of concentric shells, one of which contained the magnetic poles”, and that Halley’s model of internal terrestrial structure was “rather similar to Hooke’s (24). David Oldroyd has likewise claimed that “... Hooke’s theory was thought sufficiently adequate to furnish a model for the structure of the Earth by Halley in 1692, in his explanation of the Earth’s magnetic poles (25). There are however objections to this view.
Hooke’s model of the Earth’s interior was layered like an onion, and hardly resembled that developed by Halley (26). There is indeed no single feature which the two views shared in common. Hooke’s model lacked any air-gaps between its strata, or subterranean spheres, or principle of differential rotation, or multiple magnetic poles; nor had it any mechanism for a rotation of magnetic poles around the geographic poles. Only, in Hooke’s view, in the Earth’s pristine condition may it once have had spherical concentric shells of matter, but earthquakes had long since disrupted that ideal scheme. Hooke did not locate the Earth’s magnetic power in a specific shell as Kubrin claimed, but rather left it diffusely spread through the “magnetical core or Magnetical Globe of the Earth (27). These views appeared in Hooke’s posthumously published essay “On earthquakes”. In 1674 Hooke gave his opinion to the Royal Society that the Earth’s magnetic poles were rotating around the Earth’s axis, but this was accompanied by no suggestion as to how such an effect could be produced: he never theorized, as did Halley, as to what might produce a periodic rotation of the magnetic poles. For Hooke, earthquakes were an instrument for the transformative processes that he envisaged through the Earth’s geological history, whereas on Halley’s model they presented a difficulty which required accounting for. It therefore appears that not one of the highly distinctive characteristics of Halley’s model came from Hooke. Halley’s theory was original, and derived not from the lectures of Hooke but rather from the error in lunar mass of Newton’s Principia.
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