* * *
I.The Latin for screw is cochlea, which is Greek for “snail” or “snail shell.” The Latin for vine is vitis, which is the root of the French word vis, or screw, whence the English vise.
II.The technology of olive and grape presses was identical.
CHAPTER SEVEN
Father of the Screw
HERO OF ALEXANDRIA was a Greek. I had been taught that mechanical expertise was the preserve of the Romans, who invented the arch and the dome, never mind the auger and the plane. The Greeks were philosophers and artists. As a student, I had been to Greece, climbed the Acropolis, and visited museums. But, like many people, I had misinterpreted what I had seen. “So little has come down to us from the Greek Miracle, decisive for the birth of our sort of civilization, that we have become used to making a great deal of the things we have,” wrote Derek J. de Solla Price, a Yale professor of the history of science. “Preservation has been highly selective so that we tend to see the Greeks in terms of only the more indestructible masses of building stone, statuary, and ceramic together with coins and a few grave goods that are the main holdings of our museums and archaeological sites.”1 Indeed, the material evidence for Greek mechanical devices is so scant that, according to Price, it was long thought that the Greeks simply did not use complex machines, and that the surviving written descriptions of machines, by authors such as Hero, were merely speculative.
This belief was altered by a momentous discovery. Like many archaeological finds, it came about largely by chance. In 1900, two boats belonging to sponge fishermen were crossing the strait that lies between Crete and the Greek mainland and were swept off course by a squall. They sought shelter in the lee of an uninhabited islet called Antikythera. When the storm abated, the divers explored the unfamiliar waters, looking for sponges. Instead, at a depth of 140 feet, they discovered the remains of an ancient ship surrounded by scattered bronze and marble statues. They reported their find to the authorities, who organized an archaeological expedition.
The pottery dated the shipwreck between 80 and 50 B.C. The vessel appeared to have been a trader, sailing from somewhere in Asia Minor—perhaps the island of Rhodes—and bound for Rome. The salvaged material included many fragments encrusted with two thousand years of debris. The fragments were set aside while the archaeologists turned their attention to restoring the statues. Occasionally, the restorers went through the debris hoping to locate a missing piece of statue. Eight months into the work, during one such search, they made a startling discovery. One of the encrusted lumps had split apart, probably as the ancient wood inside shrank after being exposed to the atmosphere. The break revealed not a piece of statue but several corroded and crumbling bronze disks with inscriptions, as well as the traces of what appeared to be gearwheels. The mechanical device, whatever it was, had been contained in a wooden case about eight inches high, six inches wide, and four inches thick.
Preliminary cleaning revealed that the so-called Antikythera Mechanism was a machine of great complexity with many interlocking gearwheels. However, the heavy calcareous accretions on the fragile corroded fragments, many of which were fused together, made accurate reconstruction difficult. Some archaeologists believed that it was an ancient astrolabe; others argued that it was too complicated to be a navigation device and had to be some sort of clock. Since the oldest evidence of geared clockwork in Muslim and Chinese astronomical machines was no earlier than about A.D. 1000, to many scholars it seemed outlandish to suggest that the Greeks had this technology a thousand years earlier.2 Some argued that the mechanism was not ancient at all and had to be part of a later shipwreck on the same site. The last claim, at least, was laid to rest when it was ascertained that the disks were definitely bronze, a material used only in ancient times, more modern instruments being made of brass.
General plan of all gearing in the Antikythera Mechanism.
Decades later, as cleaning techniques improved, more of the inscriptions were deciphered and more of the mechanism was revealed. Yet the purpose of the machine remained a mystery. In 1959, Derek J. de Solla Price, who had been studying the Antikythera Mechanism, published a cover article in Scientific American titled “An Ancient Greek Computer.” He speculated that the device was used to calculate the motion of stars and planets, which made it an ancient forebear of De’Dondi’s planetary clock.3 Since the first known mechanical clock dated from the fourteenth century, this again brought counterclaims that such sophisticated technology could not have belonged to the ancient Greeks, and that the mechanism must be of later vintage. In 1971, Price and his Greek colleagues began to examine the fragments using the then new technology of gamma-radiographs and x-radiographs. They discerned layers of the mechanism previously hidden within the encrusted fragments. The final part of the puzzle fell into place when a missing crucial piece was found in the museum storeroom. It was now possible to reconstruct the machine.
According to Price, “The mechanism is like a great astronomical clock without an escapement, or like a modern analogue computer which uses mechanical parts to save tedious calculation.”4 The front dial is inscribed with the signs of the zodiac, and a slip ring shows the months of the year; two back dials, one with three slip rings, one with four, indicate lunar and planetary phenomena. Inside, the movement consists of more than thirty interlocking toothed gearwheels assembled with pins and wedges—no screws. Most of these wheels are simple circular gears that transmit and modify rotary motion, the triangular teeth of one gear engaging the teeth of the other. Price also discovered a more complex set of gears that compound two different rates of revolution—the sidereal motions of the sun and the waxing and waning of the moon—to produce the cycles of the so-called synodic month. This is, in fact, the first known example of a differential gear. The differential in the axle of automobiles, which divides power between the driving wheels and allows the inside wheel to travel a shorter distance smoothly when the vehicle is turning a corner, was invented in 1827; the differential gear in the Antikythera Mechanism was made two thousand years ago. “It is a bit frightening to know that just before the fall of their great civilization the ancient Greeks had come so close to our age,” writes Price, “not only in their thought, but also in their scientific technology.”5
The Antikythera Mechanism is the only complex mechanical instrument to survive from antiquity, yet we know that it was not unique. A similar device is described by Cicero, who witnessed a demonstration of a “celestial globe” in the first century B.C. “When Gallus set this globe in motion, it came about that the moon was as many revolutions behind the sun on the bronze instrument as in the heavens themselves, and therefore there was that same eclipse of the sun in that sphere, and the moon then met that point, which is the earth’s shadow.” Cicero was impressed. “I decided then that there was more genius in that Sicilian than human nature seems able to encompass.”6 “That Sicilian” was Archimedes, the builder of the celestial globe, who had died about one hundred and fifty years earlier. Archimedes’ globe was famous in the ancient world; it is also referred to by Plutarch and Ovid. Even eight hundred years after Archimedes’ death, Claudianus wrote a poem in which Jupiter was mocked by the “skill of an old man of Syracuse [who] has copied the laws of the heavens, nature’s reliability, and the ordinances of the gods.”7 None of these authors provided technical details, however. We know from ancient references that Archimedes himself wrote a treatise titled On Sphere-Making, but it has long been lost. Price speculates that Archimedes probably used a complicated gear train of the type found in the Antikythera Mechanism, which appears to be a later copy of his celestial globe.
Archimedes was a citizen of Syracuse, a wealthy Greek city-state on the island of Sicily. He was born about 287 B.C., the son of an astronomer, and was sent as a young man to study mathematics in Alexandria with the successors of the great Euclid. On his return to Syracuse, he devoted himself to science. He became the foremost mathematician of the ancient world, devising a variety of proofs in both plane
and solid geometry, including describing the geometry of the spiral. He wrote several treatises on the equilibrium of planes and established the mathematical foundation for the science of mechanics. In addition, he single-handedly invented the science of hydrostatics, the branch of physics that deals with fluids at rest and under pressure.
Archimedes left instructions that his sepulchral column include a depiction of his favorite proposition: the calculation of the exact ratio between a sphere and the cylinder that circumscribed it. He died at the age of seventy-five. One hundred and fifty years later, Cicero, while serving as Roman administrator of Sicily, sought out the tomb and, finding it neglected, had it restored. Cicero anticipated the interest in Archimedes by later historians such as Diodorus Siculus, Livy, and Plutarch. Of course, they wrote three hundred years (in the case of Plutarch, four hundred years) after the fact; by then, all that was left were stories. One of these concerned Archimedes’ death. During the Second Punic War, Syracuse was attacked by the Roman army, and after a two-year siege the city fell. According to Plutarch, the victorious Roman general, Marcellus, who was an amateur mathematician, sent a soldier to fetch the renowned Archimedes. “As fate would have it, he [Archimedes] was intent on working out some problem with a diagram, and having fixed his mind and his eyes alike on his investigation, he never noticed the incursion of the Romans nor the capture of the city,” Plutarch writes. “And when a soldier came up to him suddenly and bade him follow to Marcellus, he refused to do so until he had worked out his problem to a demonstration; whereat the soldier was so enraged that he drew his sword and slew him.”8 The remorseful Marcellus is said to have personally erected the mathematician’s tomb. He also appropriated two of Archimedes’ celestial globes, one of which later came into the hands of the astronomer Gallus, who showed it to Cicero.
The most famous story told about Archimedes concerns his solution of the so-called wreath problem. Hieron, the king of Syracuse, commissioned a gold wreath as an offering to the gods. He provided gold to the jeweler, who in due time delivered the finished wreath. Hieron suspected that the gold had been diluted with silver, but could not prove it. The wreath was a consecrated object and could not be tampered with, so a chemical assay was out of the question. Since the goldsmith refused to confess, the king turned to Archimedes. The mathematician pondered the matter and devised a simple experiment. He weighed the wreath, and immersed similar weights of silver and gold in a vessel of water, measuring how much water each displaced. He discovered that silver displaces more water than gold (the specific gravity of silver is almost half that of gold). Since the immersed wreath caused more water to overflow than the equivalent weight of gold, he deduced the presence of silver and proved that the wreath was indeed impure. According to legend, the idea for the water experiment came to Archimedes as he was plunging himself into a tub in a public bath. Seeing the water overflowing triggered something in his mind. “Transported with joy, he jumped out of the tub and rushed home naked,” writes Vitruvius, “crying out in a loud voice, ‘Heure-ka! Heure-ka!’ [I’ve found it! I’ve found it!].”9
Plutarch wrote that Archimedes “regarded as sordid and ignoble the construction of instruments, and in general every art directed to use and profit, and he only strove after those things which, in their beauty and excellence, remain beyond all contact with the common needs of life.”10 Yet there is no doubt that the mathematician had a mechanical bent, no less than Hero or Maudslay. Archimedes’ reputation for cleverness and ingenuity was legendary. The Romans named a popular puzzle, which consisted of various-shaped pieces of ivory that had to be assembled into a square, Loculus Archimedius in his honor. His cleverness manifested itself in many practical inventions, all still in use: the compound pulley, whose several sheaves increased lifting power and allowed a single man to raise heavy weights; the windlass, a rope wound around a drum, which was used as a hoisting device aboard ships and in mines; and the ancestor of the balancing weighing scale, the steelyard. In addition to the celestial globe, he is said to have built a water clock and a hydraulic organ in which air was compressed by water.
Like Leonardo and Ramelli, Archimedes served as a military engineer. During the siege of Syracuse he was called on to build defensive weapons. He designed catapults that threw rocks weighing five hundred pounds, and complicated underwater obstacles that capsized ships. His most renowned weapon was a mirror that beamed the sun’s rays and set the attackers’ ships on fire. To prove the practicality of what had long been considered merely a colorful legend, in 1973, a Greek engineer named Ioannis Sakas built a working version of the ancient ray gun.11 He used seventy bronze-coated mirrors, which he aimed at a tarred plywood cutout of a ship. At a distance of 165 feet, approximating the “bow-shot” that is mentioned in the classical text, it took only a few minutes for fire to break out.
In 1981, the redoubtable Sakas tested another Archimedean invention, the architronito, or steam cannon. This device was credited to Archimedes by Leonardo da Vinci, whose sketchbooks show a gun barrel with a breech encased in a heated firebox. When water is released from a cistern into the white-hot barrel, the resulting steam creates sufficient pressure to eject the cannonball. Leonardo wrote that “this machine has driven a ball weighing one talent [about twenty pounds] six stadia [about three thousand feet].”12 Sakas’s scale model successfully fired a cement-filled tennis ball a distance of two hundred feet.13
According to Plutarch, after Archimedes had written a treatise titled “To Move a Given Weight by a Given Force,” in which he claimed that any weight could be moved, Hieron challenged the mathematician to prove his assertion by moving a beached ship loaded with freight. Archimedes set up his apparatus, attached a line to the ship, “and then drew it along, smoothly and evenly as if it were floating in water, not with great labor, but sitting down at a distance.”14 It was on that occasion that he made his famous claim: “Just give me somewhere to stand, and I shall move the earth.”15 How could Archimedes move a vessel weighing as much as seventy-five tons? According to Plutarch, it was done with a compound pulley; a description by a Byzantine historian mentions a three-sheaved pulley; and Athenaeus, a Greek historian, writes that Archimedes used an endless screw. A. G. Drachmann suggested that it is not unreasonable to assume that the ship was moved by a combination of these machines. He calculated the mechanical advantage of a pulley of five sheaves, pulled by a windlass that was powered by a combination of endless screws, would be 1:125,000.16 That is, one pound of force on the rope would translate into a pulling force of more than sixty tons. Even assuming losses for friction, Drachmann argued, Archimedes alone—the different versions are in agreement on this point—could easily move the heavy ship a small distance.
Who was the inventor of the endless screw? Some modern historians credit Archytas of Terentum, a Pythagorean philosopher who lived at the time of Plato, around 400 B.C.; others point to Apollonius of Perge, a younger contemporary of Archimedes’.17 Drachmann champions Archimedes himself, citing not only Athenaeus’ story of moving the ship, but also quoting Eustathius, a Greek scholar who wrote, “Screw is also the name of a sort of machine, which was first invented by Archimedes.”18
Drachmann’s claim is all the more plausible since Archimedes’ name is associated with another kind of screw, the water screw, a device for lifting water. The water screw consists of a giant screw about one foot in diameter and ten to fifteen feet long, encased in a watertight wooden tube. The tube, whose ends are left open, is installed at a low angle, with the lower end immersed in water. As the entire apparatus turns, powered by a person walking on cleats fastened to the exterior, the water entering the lower end is moved up by the helical partitions—or threads—of the screw and emerges at the top. The water screw turns slowly, but its capacity is substantial (the lower the angle, the greater the flow), and its mechanical efficiency has been estimated to be as much as 60 percent, which compares favorably with later water-lifting devices such as waterwheels and bucket conveyor belts.I,19
&n
bsp; The oldest references to the water screw, in the second century B.C., all credit Archimedes with the invention. According to Diodorus, Archimedes invented the water screw while he was a young man in Alexandria.20 That makes sense. The device is ideal for agricultural irrigation in Egypt: unlike large waterwheels, it can easily be moved from place to place; its lift is not great, but sufficient for the flat delta; and the simple design—there are no moving parts—resists clogging by the silted Nile River water.
Water screw technology spread from Egypt throughout the Mediterranean. Water screws were used for irrigation, but they had other applications. Archimedes is said to have used them to empty bilge water from one of Hieron’s huge ships. Water screws were also used by the Romans to lift water in municipal water systems, and to pump out mines. Several well-preserved wooden water screws were discovered in the early 1900s in ancient Roman copper mines in Spain.21 The twelve-foot-long tubes, approximately one foot in diameter, are wrapped in pitched cloth and strengthened with rope; inside, the helical partitions are made of laminated strips of wood, glued and attached with copper nails. Four such water screws in series could lift water a vertical distance of about twenty feet. Diodorus describes how “with constant pumping by turns they throw up the water to the mouth of the pit and thus drain the mine; for this engine is so ingeniously contrived that a vast quantity of water is strangely and with little labor cast out.”22
One Good Turn: A Natural History of the Screwdriver and the Screw Page 8