and the Burning Mirror
engraving shows Archimedes using a large, parabolic mirror
to make his attack, but it seems more likely he would have
employed a number of smaller flat mirrors.
Probably no ancient tale has raised as much controversy
as the story of the Greek inventor Archimedes using a giant mirror,
or set of mirrors, to set fire to Roman ships attacking his home
city of Syracuse in 212 B.C.. Did it actually happen? Scientists
and scholars have come down on each side of this question. Sometimes
their conclusions are based on the historical record (or lack
of it), sometimes the researchers have actually tried setting
up the test themselves using real mirrors and mock ships. In 2010,
U.S. President Obama even challenged the hosts of the TV show
Mythbusters to test the problem and come to a definitive
conclusion. They did, but was their answer the correct one?
Let's start our exploration of the subject by going
back to an account of what Archimedes supposedly did. Zonares
and Tzetzes writing in the 12th century quoted from an earlier
work (now lost) called the Siege of Syracuse, said:
When Marcellus [The Roman General] had placed
the ships a bow shot off, the old man [Archimedes] constructed
a sort of hexagonal mirror. He placed at proper distances from
the mirror other smaller mirrors of the same kind, which were
moved by means of their hinges and certain plates of metal. He
placed it amid the rays of the sun at noon, both in summer and
winter. The rays being reflected by this, a frightful fiery kindling
was excited on the ships, and it reduced them to ashes, from the
distance of a bow shot. Thus the old man baffled Marcellus, by
means of his inventions.
It is clear that, at least on the surface, the method
described seems plausible. Plenty of school children have used
a magnifying glass to concentrate the sun's rays onto a small
piece of wood, setting it on fire. The same thing can be done
with a small parabolic mirror which can take the sun's rays and
reflect them onto a small point. The writers suggest that Archimedes
did this on a grand scale with a huge mirror focusing it on ships
anchored "a bow shot" from the wall of the city (perhaps 500 feet).
The ability of mirrors to concentrate the sun to
obtain very high temperatures is well-known. A few years ago the
U.S. Department of Energy, along with a group of businesses, constructed
a solar "power tower" station in the Mojave Desert near Barstow,
California, to demonstrate the feasibility of harnessing this
kind of energy. The station consisted of an array of mirrors that
reflected the sun to a tower in the center of the field. At the
top of this tower was a large target that used molten salt to
absorb the heat and transfer it to a water boiler on the ground.
The boiler created steam which then ran turbines and generators
like a regular power plant to make electricity. The plant could
produce 10 megawatts by using almost 2000 mirrors that would move
under computer control, tracking the sun and reflecting its light
to the tower.
Solar II power plant near Barstow, CA, used almost 2000
mirrors and the heat of the sun to make 10 megawatts of
It's easy to imagine focusing those mirrors on a
wooden ship instead of the tower and setting it ablaze. The target
at the top of the tower can reach temperatures greater than 1,000
Fahrenheit, well about the autoignition point of most wood. However,
each of the installation's mirrors was huge (430 square feet-which
is a mirror over twenty feet long by twenty feet wide) and there
were almost 2000 of them, probably not something Archimedes could
have arranged given the technology at the time.
So the question isn't whether the sun could provide
enough heat, it is whether Archimedes could have built a mechanism
with the tools he had available at the time to concentrate enough
sunlight to set the deck of a wooden ship on fire.
Many illustrators have pictured the old inventor
using a single parabolic mirror a few feet in diameter to do the
job. A parabolic mirror has a curved shape that will focus parallel
light rays coming from a light source (like the sun) onto a single
point (called the focal point) which is a set distance from the
mirror (the focal length).
The problem with this scheme is that a single, solid
mirror has a set focal length. This means that the ship you want
to set to fire must be precisely the focal distance away from
your mirror for such a scheme to work. Any closer or farther and
the light isn't concentrated on a small point, but is instead
spread out over a larger area. This in turn means that the resulting
temperature will probably not be high enough to start combustion.
Since Archimedes couldn't control how far away Marcellus
parked his ships, he couldn't anticipate where to set the focal
point when he built his mirror. One way to get around this problem
is to build such a large mirror that even if the focus point isn't
precisely on the ship you still concentrate enough light on it
to get it burning. The problem with this is that a single mirror
the necessary size would be extremely hard to move and control
with the primitive equipment Archimedes had.
parabolic mirror focuses parallel light rays to a single
point concentrating their power.
Another problem with a large parabolic mirror is
that the farther the focal point is away from the mirror, the
less energy you can focus on it. For example, a one foot diameter
parabolic mirror with a ten foot focal length can concentrate
light from the sun by 10 times. However, a one foot diameter parabolic
mirror with a one hundred foot long focal length would not concentrate
sunlight at all. If the Roman ships were any distance from the
mirror, say 500 feet, Archimedes would need a parabolic mirror
at least 10 feet in diameter to just double the normal heat that
the target would get from the sun itself. This would be far less
than what would be needed to get a fire going.
All this seems to make it much more likely that
Archimedes instead used a number of small, flat mirrors instead
of one, big, parabolic one. Each of the mirrors could be used
to focus sunlight to a small spot on the target. Together they
would form a shape roughly the same as one huge parabolic mirror.
To do this he might have equipped hundreds of soldiers with mirrors.
Working together, with each man adjusting his own mirror, they
might have been able to focus enough light on a single point on
a ship to get it burning.
This approach is exactly what a number of attempts
to reproduce Archimedes achievement have used. Apparently in 1973
a Greek scientist, Dr. Ioannis Sakkas, became curious about whether
Archimedes could really have used a "burning glass" to destroy
the Roman fleet and set up an experiment involving 60 Greek sailors
each using an oblong 3' by 5' flat mirror to focus light on a
wooden rowboat 160 feet away. According to sources he had no problem
getting the wood to catch fire very quickly.
In 2009, as an exercise in design, a class at MIT
decided to try and reproduce the Archimedes weapon using 127 one
foot by one foot mirrors. They were successful in getting a mocked
up ship made out of red oak to start burning after ten minutes
of exposure to direct sun. Their experience, however, showed some
flaws with the idea of using this as a weapon.
The problem with using individual mirrors is that
each of them has to be aimed at the target (a specific point on
the ship) at their own particular angle from the sun. If you handed
one person a mirror and asked him to reflect the sunlight to a
location, it is easy for him to do. He simply uses the bright
spot formed by the mirror's reflection as a guide to aiming at
the target. If you give one-hundred people mirrors and try to
do the same thing, however, you have chaos. One hundred mirrors
means one hundred bright spots and nobody is sure which spot is
multiple flat mirrors could concentrate light on a ship,
but could they really get it hot enough to burst into
Lee Krystek, 2011)
With some training and perhaps the use of an aiming
device this problem can be minimized. A more difficult problem
to solve is the length of time needed by the MIT crew to get a
fire going. Ten minutes is a long time to keep your attention
focused on aiming a mirror at distant point while the chaos of
battle is going on around you. Plus the length of time necessary
to get anything burning could easily be stretched out even further
if the enemy took the simple defensive measure of splashing water
on your target area to cool it down.
In 2010 the Mythbusters television show attempted
use 500 flat mirrors controlled by 500 volunteer middle and high
school students to reproduce the burning mirror legend. Despite
an hour of focusing the sun on a sail (which should have had an
ignition point of only about 500 degrees Fahrenheit) they could
only get the temperature up to about 230 degrees. Even more significant
is that they used modern silver mirrors do this. Polished bronze
mirrors, more typical of what would have been available in Archimedes
day, would have been 30 percent less efficient, resulting in a
The Mythbusters conclusion was that although
in theory setting a ship on fire with mirrors might be possible,
it seems an unlikely method to be used in battle. Jamie Hyneman,
one of the Mythbusters hosts who was stationed aboard the
mock ship during the experiment, observed that while the ship
did not burst into flame he found the dazzling brightness of the
mirrors disconcerting and suggested that Archimedes might have
simply used them to confuse the enemy and disrupt their ability
to see their opponents.
Others have suggested that while the mirrors might
not have been able to set the ships on fire, they might have been
very effective in harassing the crew of the ship. Having a light
beam focused on you that could raise your skin temperature to
over 200 degrees would be extremely unpleasant.
Accounts without Mirror
fire was thought to be a form of sticky petroleum that would
be set on fire. It could be delivered in a jug, or sprayed
on with a siphon as shown here.
Given that it seems that mirrors are an impractical
way to set a ship on fire, perhaps it makes sense to look at the
accuracy of documents that claim that Archimedes actually did
this. The quote we looked at earlier was written 1,400 years after
the fact. It says that it is quoting an earlier work, but that
early work is now lost so we can't really see the source material
Writers contemporary to the era, like Polybius,
Livy, and Plutarch, never mention the use of mirrors to set ships
on fire, though they do discuss other of Archimedes defensive
devices. In particular they tell about "The Claw of Archimedes"
which was apparently a crane that stuck out far over the city
wall and would use a grappling hook to catch the Roman ships,
lift them and overturn them.
Two second century A.D. writers, Lucian of Samosata
and Galen of Pergamon, do say that Archimedes set fire to the
Roman ships, but don't say exactly how. This has led to speculation
that the inventor may have employed a different method than mirrors:
Exactly what Greek fire was is a bit of a mystery
to historians today. Sources tell us it would burn on water and
some sources even tell us it would be ignited by water. A number
of researchers, however, suspect that Greek fire was actually
a thick, sticky form of petroleum. While such a material wouldn't
actually burst into flame on contact with water, neither would
water work as a particularly effective extinguishing agent, and
might actually spread the flames. The easiest form of delivery
would have been a pot filled with the stuff that has a lit wick
sticking out of the top. When cast onto a ship, the pot would
break open spilling out the liquid which would be set on fire
by the flame. The arrangement would act precisely like an ancient
version of a molotov cocktail.
sets fire to a ship using a huge mirror made up of many
flat segments in this silent film from 1914.
Another of Archimedes defensive designs supposedly
deployed during the battle was a form of catapult. It would be
easy to envision one of these machines pelting the Roman ships
with pots of Greek fire and reducing them to ashes. If at the
same time Archimedes was also employing mirrors to confuse and
blind the enemy, an observer of the battle might be confused about
what had caused the fires.
So did Archimedes set ships on fire at the Siege
of Syracuse? We may never be sure of the truth. He certainly could
have attempted it, though many of the facts we have today seem
to suggest he might have not been as successful as the legend
says. The history of the era probably supports this conclusion.
If the Romans had found burning mirrors an effective defense during
the battle, there is little doubt that they would have figured
out how to add such a weapon to their own arsenal, which they
A Partial Bibliography
Greek Science in Antiquity by Clagett,
Marshall, Dover Publications, 2001
Archimedes and the Burning Mirrors of Syracuse
by D.L. Simms, Technology and Culture Vol. 18, No. 1 (Jan., 1977)
TV Review: MythBusters 8.27 - President's
Challenge by Ann Wells, Fandomania, http://fandomania.com/tv-review-mythbusters-8-27-presidents-challenge/
Copyright 2011 Lee Krystek.
All Rights Reserved.