Hollow Earth

Hollow Earth

Halley, 1692; Symmes, 1818

The sectional view, as the doctrine's supporters drew it: a shell, further shells within, and an opening at each pole wide enough to sail into without noticing. The numbered rays are sightlines, and are the part of the diagram doing the arguing.

Proposed by	Edmond Halley, 1692[1]
Proposed because	The Moon seemed too dense[2]
Shells	Three, and a core
Interior	Luminous; possibly inhabited[3]
Aurora explained as	Gas escaping from inside
Openings added by	Symmes, 1818
Width of an opening	About 1,400 miles
Disproved by	Weighing a mountain, 1774
Abandoned	Considerably later
Expeditions produced	One, and it was real

The hollow Earth is the proposal that the planet is a shell, or a set of nested shells, enclosing a habitable space. In the form that matters it was published on 19 October 1692 by Edmond Halley, then Clerk to the Royal Society and later Astronomer Royal, in the Philosophical Transactions, alongside the work on magnetic variation that occasioned it.^[1]

It is worth being exact about why he proposed it, because the reason is not what anyone expects. Newton had estimated the density of the Moon at about nine-fifths that of the Earth. That estimate was wrong, and it is Newton's error rather than Halley's. Halley, unwilling to doubt the Principia and obliged to make the Earth lighter than it looked, took the mass out of the middle.^[2]

What Halley proposed

The scheme was specific. An outer shell about five hundred miles thick; within it two further concentric spheres; within those a solid core. Atmospheres between the shells, and each shell carrying its own pair of magnetic poles and turning at its own rate.

The last part is the point. Halley was trying to account for the secular variation of the compass, the slow drift of magnetic declination that had been vexing navigators for a century and that no fixed arrangement of poles could produce. Spheres rotating at different speeds, each magnetised, give exactly a slow drift. It is a mechanism, it is testable in principle, and it was the best available explanation of a real and expensive problem.

He went further in a way that has done his reputation no good, supposing the interior atmospheres luminous, allowing that they might be inhabited, and suggesting that the aurora was that inner light escaping at the poles.^[3] None of this was idle: a luminous interior is what the hypothesis requires if anything is to be seen there, and the aurora was itself unexplained. The scheme has one defect, which is that the Earth is not built that way. It has no others.

The weighing

The hypothesis was not argued to death. It was weighed.

Pierre Bouguer made the first attempt in Peru in the 1730s, measuring how far a plumb line was pulled aside by Chimborazo, and got a result too rough to settle anything but pointing the right way.^[4] The decisive work was done at Schiehallion in 1774, where Maskelyne measured the mountain's deflection of the vertical and Charles Hutton reduced the survey.

Hutton's number was the mean density of the Earth, and it came out at something like four and a half times that of water, against about two and a half for the rocks anyone can pick up. The interior is therefore very much heavier than the surface, not lighter and certainly not absent. There is no room in the arithmetic for a cavity.^[5]

That is the whole refutation, and it was available from 1774. Everything after that date is a different activity conducted with the same vocabulary.

Symmes

The globe as Symmes's followers engraved it, opened at the northern rim to show the shells within. The land is drawn with the care of a real map right up to the edge of the hole.

John Cleves Symmes Jr. was an American army captain and trader who added to the doctrine the feature it is now remembered for: openings at the poles, about fourteen hundred miles across, through which one might sail into the interior.

On 10 April 1818 he published Circular No. 1, and posted it, at considerable personal expense, to every notable government, reigning prince, legislature, city, college and philosophical society he could compile an address for. It declared the Earth hollow and habitable within, and closed by pledging his life in support of the truth of it and offering to go and explore the hollow if the world would fund him.^[6]

The world did not. He lectured for eleven years instead, acquired a body of followers who called him the Newton of the West, and died at Hamilton, Ohio in 1829 without having been north of Ohio in any relevant sense.

The expedition

Here the story does something unusual, and it is the reason this article is not merely a list of errors.

Symmes's cause was taken up by Jeremiah N. Reynolds, a far better publicist, who spent years agitating for a national expedition to the southern ocean. The agitation worked. The United States Exploring Expedition of 1838 to 1842 sailed as a direct consequence, surveyed some fifteen hundred miles of the Antarctic coast, and returned with charts, specimens and observations that founded a good deal of American science.^[7]

Reynolds was left behind when it sailed, after quarrelling with the Navy. The expedition found no opening, because there is none, and it found the continent, because there is one.

After

The doctrine did not stop. Reed, Gardner and others carried it into the twentieth century, and Cyrus Teed inverted it altogether in 1869, arguing that we live on the inside of the shell and the universe is enclosed by us, a position that has the merit of being harder to disprove by weighing.

What survives in circulation today is Symmes's version with the reasoning removed. The polar openings are retained; the secular variation of the compass, which was the entire point, is not mentioned; and the hypothesis is described as a thing the scientific establishment refuses to examine. It was examined by the Astronomer Royal who proposed it, by a Royal Society expedition to Peru, by a mountain survey in Perthshire, and by a national expedition to Antarctica that was mounted largely at the doctrine's own urging. Few ideas have been taken more seriously by the people alleged to be ignoring it.

See also

• Edmond Halley – who proposed it, for a reason, and did not live to see it weighed
• The Schiehallion experiment – the weighing that settled it, and the contour line it produced
• Deflection of the vertical – the quantity that weighing was made of
• The figure of the Earth – the shape of the outside, measured the hard way
• Moon – the body that was weighed wrongly, and the whole reason for the cavity
• Isaac Newton – whose lunar density is the mistake at the bottom of all this
• Flat Earth – the other cosmography that outlived its own refutation
• The Final Experiment – a later expedition to the same latitudes, on a similar errand
• Vulcan – another body proposed for a good reason and abolished by a better measurement
• Cyrus Teed – who put the cavity on the outside and us on the inside of it
• Sphere – the surface in question; this doctrine concerns which side of it is furnished

References

1. ^ E. Halley, "An Account of the Cause of the Change of the Variation of the Magnetical Needle, with an Hypothesis of the Structure of the Internal Parts of the Earth", Philosophical Transactions, 19 October 1692. The title is the argument: the hollow Earth arrives as the second half of a paper about compasses.
2. ^ Newton's figure for the lunar density, about nine-fifths the Earth's, is nearly three times the true value: the Moon's density is about 0.61 of the Earth's, not 1.8. He had it from tidal data taken in the Bristol Channel, and it put the lunar mass at about a twenty-sixth of the Earth's where the true figure is an eighty-first. It is the single most consequential arithmetical slip in this encyclopedia, and it produced a doctrine that is still being defended three centuries later by people who have never heard of it.
3. ^ The luminous interior and its possible inhabitants are Halley's, and are usually quoted to make him ridiculous. They follow from the hypothesis rather than decorating it.
4. ^ Bouguer's Peruvian measurement was the first attempt to weigh the Earth against a mountain, and he thought his own result unsatisfactory. He was right, and it was still enough to make a hollow interior unlikely.
5. ^ See the Schiehallion experiment. The modern figure is about 5.5 times water; Hutton's 4.5 was low, and low by nowhere near enough to help.
6. ^ Circular No. 1, 10 April 1818, sent at his own cost and over his own name, with his rank and address attached. Whatever else it was, it was not anonymous and it was not hedged.
7. ^ The United States Exploring Expedition surveyed a long stretch of the Antarctic coast and is the reason a portion of it is called Wilkes Land. Its scientific collections went on to form part of the founding holdings of the Smithsonian.

External links

• E. Halley, "An Account of the Cause of the Change of the Variation of the Magnetical Needle…" – Philosophical Transactions (1692) – the founding paper
• The sectional view of the Earth showing the openings at the poles – Wikimedia Commons
• The opened globe, from Symmes's theory of concentric spheres – Wikimedia Commons
• Hollow Earth – general reference