Easter Island (2000)
PBS NOVA - Secrets of the Lost Empire, Episode 3
by PBS (http://www.pbs.org/wgbh/nova/easter)
This remote Pacific island's so-called moai statues are among the world's most enigmatic sculptures. In this section, explore an interactive map of Easter Island to find out where ancient residents quarried and moved the famous monoliths. Also, follow recent attempts by NOVA and others to transport moai overland.
This is the story of a team of archaeologists and a 75-person crew who sought to unravel a central mystery of Easter Island: how hundreds of giant stone statues that dominate the island's coast were moved and erected. For one month, the team struggled to raise a 10-ton moai, using only the tools and materials available to the ancient Easter Islanders.
The giant statues of Easter Island are one of archaeology's greatest enigmas. Nearly 1,000 of the massive, haunting human sculptures rise from the windswept grasslands of this tiny volcanic island, almost 1500 miles from the nearest inhabited land. How and why did the ancient islanders transport the statues - some weighing 80 tons - across miles of rugged terrain and then erect them on ceremonial platforms?
Countless ingenious theories have been proposed. Thor Heyerdahl, the famous Norwegian explorer, attempted to "walk" a statue along by rocking it from side to side in an upright position. Geologist Charles Love fastened a replica to a wooden sledge and tried pulling it on rollers. But neither met with much success: Love's statue came crashing to the ground while Heyerdahl's broke in transit.
The failure of these experiments inspired Jo Anne Van Tilburg, a leading authority on Easter Island, to seek her own solution to the riddle. Drawing on her painstaking inventory of the statues, she built a computer model of Easter Island's terrain, created a digital statue, and experimented with different transportation and erection techniques in cyberspace. This led her to a solution that seemed flawless. But would it work outside the computer?
To test her theory, NOVA casts a 15-ton concrete replica of a typical Easter Island statue. With the help of 70 Easter Islanders, the statue is then hauled across over a mile of the ancestral terrain, and the challenge of erecting it begins. What seemed so straightforward in the computer now reveals hidden complications that plunge the team into rival theories and disagreement. As the mystery of the builders' methods deepens, NOVA explores the profounder challenges that Easter Island has always posed: Who were the ancient islanders? Where did they come from? And what went wrong with their unique and exotic civilization?
Original broadcast date: 02/15/2000
Topic: anthropology/ancient, archeology, technology/engineering
Vince Lee's Uphill Battle
by Vincent R. Lee
In the spring of 1998, I was asked to take part in the filming of the NOVA film "Easter Island," during which experts examined hand methods for moving and erecting moai, the huge stone statues for which the island is famous.
Dragging the moai At the Easter Island site, the NOVA team drags the concrete moai on its specially designed sled.
Moving a Moai The plan was to move a nine-ton concrete moai overland onto a fieldstone "ahu" or platform and leave it standing erect with a stone topknot or "pukao" balanced on its head.
Most ahus are effectively seawalls, accessible only from their inland sides. Long columns of pullers could not possibly have dragged their moai into place atop these platforms without falling off the back into the ocean.
Fortunately, the NOVA ahu was inland and a large gang of pullers on the open ground beyond its "seawall" was able to drag the sled into place. Nevertheless, it was abundantly clear that the final movement up onto a coastal ahu wasn't just an unsolved detail: It was the crux of the problem, since with enough people almost anything could be dragged across open country.
A New IdeaAll of this added up to a whole new way of looking at moving big rocks. Like many others, I had been distracted by scenes from antiquity showing giant monoliths being pulled on sleds. Now I knew that the ancients must have used another method altogether in places with no room for all those long columns of pullers. Needed was a much smaller workforce able to do the same job, and as Archimedes pointed out 22 centuries ago, there was only one way to do that.
Levers were the only tools most pre-industrial people like the Rapanui had for multiplying the muscle power of a man. By clever application of the idea, one leverman might do the work of five or even 10 conventional laborers. Used to maneuver huge stones in confined places, levers would greatly reduce the work force needed and, more to the point, the space it required. In theory, everyone could push from alongside, behind, or atop the rock, eliminating the need for any pullers at all—exactly what was needed to get a large moai up onto a high, coastal ahu.
What if moving big rocks was like moving ships? Mediterranean galleys had sails for favorable winds and open water, and oars for close quarters and auxiliary power. A nautical analogy seemed especially appropriate on Easter Island. Didn't the islanders' ocean-going canoes, like the galleys half a world away, use both sails and paddles? At one point in the NOVA film, the team hauled a heavy outrigger up over the rocky shoreline on a Polynesian "canoe ladder," a frame identical to the leapfrogging sliders I'd earlier envisioned as the ideal roadway for moving moai.
In a flash, an entirely new and surprisingly simple alternative transport method came together: a lever-friendly sled pulled easily over lever-friendly canoe ladders as much as possible, but turned, rotated, or nudged slowly forward by levermen wherever necessary (see Figure 1).
Simple as the idea may seem, the devil, as always, is in the details. The leapfrogging ladders promise a smooth, easily leveled, and effectively endless working surface with as many firm purchases for the toes of the levers as possible. The latter is especially important, since slippage against the ground greatly reduces lever efficiency, a common problem unless something is done to prevent it. The sled, too, offers as many places to push against as possible but spaces them to optimize placement of the levermen.
To direct maximum forward force on the sled, a leverman's lever should never travel more than about 20 degrees of arc or go much beyond the vertical (see Figure 2). Each leverman's mechanical advantage is the ratio of the distance between the sliders and the sled's crossbars versus his shoulder height—an advantage of about three for a man standing on the ground and five if he rides the sled. If he stands atop the moai with a long lever, his advantage might be as much as seven, eight, or more, depending on the size of the statue. His added weight is much more than offset by the increased leverage.
As critical as the force urging the sled forward is the frictional force holding it back, and every effort to smooth and grease the contact surfaces reduces the required crew size. All lashings have to be neatly done and recessed or otherwise prevented from hanging up, and the sled runners need to be beveled at both ends to ride smoothly onto the sliders before and after the required 180-degree rotation of the moai. One would rotate the sled with this system much as one would rotate a rowboat or canoe, by levering one side forward and the other back. It is important that the sled runners not get parallel to the sliders and fall into the gaps between them, though levering forward onto ladders set 90 degrees from one another avoids the problem (see Figure 3).
A Hasty Experiment Having arrived at this solution, I made a small model to illustrate the idea and suggested we try a full-scale test to see if it would actually work. NOVA agreed, and using some skinny leftover poles and a roll of small diameter cord, we hastily tied up a sled and two ladders with which to try moving a "three-ton" rock promised by a local supplier. Due to a shortage of material, we ended up with about half as many sliders and sled crossbars as my design called for, but I figured it wouldn't matter with such a small load.
The rock finally arrived about 10 a.m. the final day of the shoot, weighing at least six tons and looking like a gigantic Idaho spud. Next to it, our sled and ladders seemed woefully flimsy and inadequate. The "crane" we expected for getting it off the truck and onto our sled turned out to be a two-ton swing arm hoist mounted on the truck; it proved all but useless. Instead, we rolled the boulder off the truck, shattering the truck bed's edge planks in the process, and watched our "moai" thud ominously into a shallow ditch.
Just getting the boulder out of the ditch and onto the sled was a huge project. We levered it up onto one of its "edges" and then rolled it onto our sled, simulating a moai in the prone position. The sled lay visibly crushed under the load, with one runner cracked and both pressed tightly into the mud of the ditch bottom with no sliders underneath. As the camera rolled, we began levering the grossly overloaded tangle of poles up out of the ditch and onto the first ladder, prying between the projecting crossbars on the sled and the ground.
It was horrendously inefficient. The levers "kicked out" as they approached the vertical, so that much of our force went up instead of forward. Also, as we got onto the ladder, the hastily done lashings beneath the sled's crossbars tended to hang up on the sliders. Somewhat to our amazement, the sled nonetheless moved, and in about an hour we got it nearly onto the first ladder. With time, energy, and daylight running short, however, NOVA asked us to switch gears and try rotating the sled. That proved a bit easier, since it let us turn off the fall line and across the slight grade we'd been fighting all afternoon. In a few minutes, the sled rotated about 30 degrees.
One observer watching our struggle said, "Look at it. It's a mess!" I had to agree, but for one thing: It worked. My 12-man crew had levered a six-ton rock about 15 feet in an hour and a half. Each man had moved about 1,000 pounds of rock with no help at all from pullers. If we'd done everything right, our performance would have been much more impressive, but even as it was, our rate of progress (say 80-100 feet per day) was about the same as the previous methods tested in the NOVA film with four to six times as many people. Once underway, their sled moved much faster than ours, of course, but they lost many hours re-rigging their gear between moves. Slow as our progress was, we could nevertheless have gotten our "moai" up onto its ahu without anyone working on the platform's seaward side.
But could our method move an 80-ton moai, about the largest ever moved by the Rapanui (see How Big Were They?). The problem is partly simple arithmetic. If each of the 40 or so pullers hired by NOVA to pull the replica moai exerted 100 pounds of force to move their sled, the friction coefficient (the ratio of the weight of a load versus the force needed to drag it) using sliders over rails must have been about .2 (4,000 pounds of force divided by 20,000 pounds of moai and sled weight = .2). Applying this coefficient to an 80-ton load requires 16 tons of forward force or 320 pullers at 100 pounds per person. My levermen were each moving about 1,000 pounds of load, which would cut this workforce in half (80 tons = 160,000 pounds / 1000 = 160 levermen). But how many men would it take if levers were used properly, correcting all the problems we encountered during our hurried test of the idea? There was only one way to find out.
The Sisyphus Project On December 15th, 1998, my friend Bruce Davis and I collaborated on a full-scale trial at his masonry contracting yard in Brighton, Colorado. We named our test the Sisyphus Project after the King of Corinth whom, in Greek mythology, the lord of the underworld condemned to an eternal afterlife of futilely pushing a large stone up a hill.
I had commissioned Mark Younghein, a log-cabin contractor in Longmont, Colorado, to fabricate a 10x15-foot sled and four 5x10-foot ladder sections from stout lodgepole pine logs. The sled weighed about a ton and the ladders were in four 500-pound sections for ease of carrying. The designs were otherwise similar to those shown in Figure 1, except that the sliders and sled crossbars were let several inches into the tops of the rails and runners, respectively, to achieve tight, rigid joints. This turned out to be an important detail. Any play in these joints greatly reduces the effectiveness of the levers, since part of each throw is wasted taking up the slack. Finally, we provided both long and short levers about three inches thick for people working atop the load and on the sled, respectively (see Figure 2).
Meanwhile, Davis's crew had constructed a scaled-down replica of the steepest ramp still existing on Easter Island (see Figure 4), a daunting 1:4 or 25 percent slope. His team also assembled several large blocks of white marble from which to select a load, depending on how many people turned out to help move it.
Overview of ramp Volunteers on the Sisyphus Project begin to rotate the 13-ton block atop the ramp.
We notified potential volunteers and about 30 showed up on the day of the test. Based on the idea that each person might move 1,000 pounds of rock, we played it safe and selected a 13-ton block. We abandoned initial thoughts of using water as a lubricant when the moisture either soaked into the soft pine logs or quickly evaporated into the high, dry mountain air. Instead, we greased the sled runners with lard from the supermarket on the theory that the Rapanui surely had lots of fish oil and wouldn't have hesitated to use it.
The project we set for ourselves was simple: move the rock up the ramp and rotate it at least 90 degrees with levers alone, using no more than the working surface of the ramp for the purpose. After a nervous first effort that turned out to be serious overkill, we cut the crew back to see how few people could move the sled on level ground. Eight long levers handled from atop the rock was all it took. Each person had a mechanical advantage of about seven, imparting about 700 pounds of forward force—a total of 5,600 pounds for the group. Their combined weight and that of the sled added about 3,500 pounds to the 13-ton block, so the coefficient of friction must have been about .19 (5,600 / 29,500 = .1898).
Leverers silhouetted Unlike Sisyphus, Vince Lee got what he wanted: A large block levered up a steep ramp with few people in a short period of time.
Adding people as needed along the way, we moved the load up the steepest part of the ramp with 26 people. Unintentionally, we apparently arrived at the perfect balance between friction and mobility, since the sled tended to backslide ever so slightly when forward pressure was released. Two "brakemen" easily prevented this by jamming their levers whenever we stopped. Mechanically, the system worked exactly as expected, though the animal fat was critical to its success. Whether even better lubrication would have improved performance remains an open question. Easier uphill movement might have increased the danger of downhill slippage and created problems maintaining control of the sled.
It is not known where the Rapanui rotated their moai, but to cover the worst-case possibility we decided to do it atop the ramp, in the most constricted location of all. We easily accomplished the maneuver as shown in Figure 3, except that by levering both sides ahead, the outside of the turn more than the inside, we needed no pullers for forward movement onto the turned ladder. So levers alone carried the day, and it took only about two hours to climb the 55-foot ramp and make the turn at the top.
An Open QuestionHowever feasible the scheme described here may be, we have no direct evidence that the Rapanui or anyone else in ancient times used it. Nevertheless, it is clear that the hundreds of people needed to drag the largest moai on Easter Island could not possibly have pulled them all the way up onto any of the numerous seacoast ahu there. They must have used some other, much more efficient transport method. The simple levering system proposed here is the only idea thus far proven to solve this problem, and it would have worked for the Rapanui and anyone else in antiquity faced with moving big rocks up steep hills and into small places. Did the ancient Rapanui do it this way? We may never know. But we now know a method they could have used, which is more than we knew before.
Vincent R. Lee is owner of Design Associates, Architects in Jackson Hole, Wyoming and a research associate both in Andean Studies at Berkeley and with the Museum of Man in San Diego. He has led numerous field expeditions and mapping projects in Latin America as well as various studies in pre-Columbian architectural design and construction techniques, particularly those involving megalithic masonry.
How Big Were They?
by Dennis Gaffney
Between A.D. 1000 and 1600, the people of Easter Island, a small island isolated in the vastness of the Pacific Ocean, carved about 900 massive statues from an ancient volcano. These austere-faced giants, called moai, still stand, their backs turned to the sea.
It's clear that each moai was difficult to move over the island's hilly terrain—but how much do the largest stones weigh? In this NOVA game, you can find out by comparing the weight of the largest moai to a herd of elephants.