The Economist December 18th 2021 Science & technology 63es around three times as much thrust per
unit of energy expended as a typical small
boat’s propeller can manage. And he hopes,
soon, to do even better than this. Having
demonstrated his device to America’s Of
fice of Naval Research, he has piqued their
interest. The result is a commission for a
followup, cRay, that should be lighter,
faster and yet more efficient.
Unlike Velox, which is controlled by ca
ble, cRay will be autonomous—the ulti
mate aim being to develop cooperative
swarms of craft for jobs such as mine de
tection and removal, reconnaissance and
antisubmarine patrols. From a naval per
spective, however, undulatory propulsion
may have a yetmoreimportant advan
tage. Submarines are often detected by the
noise they make, much of which comes
from the propeller and the shaft driving it.
Undulatory propulsion, moving more wa
ter at lower speed, should be quieterthan
any propeller. Nor does it involve anoisy
phenomenon called cavitation, causedby
transient gas bubbles that form in re
sponse to propeller blades’ pressure.
This matters, because Veloxlike fins
may prove to be a technology thatcanbe
scaled up to propel fullsized submarines.
As Mr Filardo observes, the largest marine
animals of all, the great whales, arefin
propelled, even if their fins are arranged
differently from Velox’s. Indeed, thebig
gest of the lot, a blue whale, can travelat
more than 20 knots, which would notdis
grace the average submarine. Previousat
tempts to scaleup finpropulsion have
failed, he says, because they have not
found the necessary compromise between
stiffness and flexibility. He reckons hehas.
Travelling waves
Even if they do not make the bigtime,na
valwarfarewise, swarms of Velox’s de
scendants might be deployed for tasks
from harvesting scallops without destruc
tive trawling to mining nodules fromthe
seabed without harming habitats—forun
dulatory propulsion does not disturbsedi
ment. In a world where the creationofnew
carbon sinks may become big business,
they might even be used to plant bedsof
seagrass on a vast scale. Craft propelledby
undulation would also have less riskof
harming swimming mammals, such as
manatees and human beings, whichsome
times get chewed up by propellers.
Mr Filardo is even looking into theidea
of merging his interests, by designinga
craft with undulating propulsion thatcan
moor itself and then recharge its batteries
from disturbances to its fins caused by
passing ocean currents. Just how farheor
others will be able to push this newap
proach to propulsion remains to beseen.
But if the engineering works, and canin
deed be scaled up, ship’s propellersmay
one day look as oldfashioned as sails.n
GeomagneticarchaeologyOne with Nineveh
and Tyre
W
hensennacherib, KingofAssyria,
sent his army to the kingdom of Ju
dah in 701bc, and had it destroy the city of
Lachish, 43km southwest of Jerusalem, he
was doing his bit for science as well. As
Yoav Vaknin of Tel Aviv University told this
year’s meeting of the American Geophysi
cal Union, held in New Orleans and online,
residual magnetism in the burntdown
buildings is helping archaeologists to date
other finds in the area. It also helps geo
physicists to chart the ups and downs of
terrestrial magnetism.
Mr Vaknin’s work at Lachish is the most
recent of a series of studies he has per
formed that were enabled by arsonous an
cient kings. The first examined a building
destroyed when Jerusalem was burned by
Babylonian troops in 586bc. According to
the biblical Book of Kings, this happened
“in the fifth month, on the seventh day of
the month, which was the nineteenth year
of King Nebuchadnezzar, King of Babylon”.
Mr Vaknin says that date is considered
reliable by biblical scholars. So he and his
colleagues were able to pinpoint the mo
ment when this building, the remains of
which were found under a car park, was de
stroyed. Those remains included pieces of
the polished upper floor and the burnt
wooden beams that once supported it.
The fire’s heat would have erased any
magnetism in the minerals of this floor.
Earth’s magnetic field then left its mark as
those minerals cooled, magnetising themanew. Assuming the fragments have not
moved since then, the alignments of their
magnetic fields will point in the direction
of Earth’s field as it was on that fateful day.
Looking for magnetic alignments in
this way was well understood when Mr
Vaknin began his investigation. But he and
his colleagues also did an experiment.
They heated samples of the fallen floor in
their laboratory and exposed them to a
magnetic field as they cooled down, thus
repeating what had happened when the
edifice was destroyed. By comparing the
resultant magnetisation with the original
one, and knowing the strength of the field
they had themselves applied, they were
able to estimate the strength of Earth's
magnetic field on the day of the sack.
They have now repeated this approach
at other sites, leading to welldated recon
structions of the magnetic fields connect
ed with the sackings of Gath, Kinneret,
Bethsaida and Ekronin, as well as Lachish
and Jerusalem, courtesy of military cam
paigns by Aramean, Assyrian and Babylo
nian kings. The dates range from about
830 bcto exactly 586bc.
That is more or less the maximum span
for which this approach is feasible. Written
accounts of earlier invasions, in the Bible
and other texts, are thought insufficiently
reliable. And after the Babylonians came
the Persians, who were, as Mr Vaknin ob
serves, “kind enough not to destroy cities”.
It is, though, an extremely useful span,
for it coincides with a hiatus in the archae
ological record called the Hallstatt plateau.
This “plateau” is a flat stretch of the cali
bration curve used for a technique called
radiocarbon dating. It is a period, from
400800bc, when, for reasons not entirely
clear, radiocarbon dating breaks down.
Samples from the time of the plateau
have hitherto been undatable within that
fourcentury span. This might now
change. Radiocarbon dating relies on mea
suring the amount of ^14 C, an unstable iso
tope of carbon, in organic materials such
as wood. Thanks to the ingenuity of Mr
Vaknin and his colleagues, and the ruth
lessness of ancient kings, the magnetisa
tion of inorganic materials that have been
exposed to heat, such as shards of pottery
from cooking vessels, offers an alternative.
Mr Vaknin’s data points are also valu
able for geophysicists who want to under
stand how movements in Earth’s core
change the planet’s magnetic field. The
period Mr Vaknin studies was one in which
this field was usually about 50% stronger
than it is today, and for short periods was
double today’s strength.
As to Lachish, history did not forget it.
Sennacherib celebrated victory by ordering
huge alabaster reliefs of his victory (one of
which is depicted above) todecorate his
palace in Assyria’s capital, Nineveh.They
are now in the British Museum.nEarth’s magnetic field illuminates
biblical history