One Good Turn: A Natural History of the Screwdriver and the Screw (12 page)

Hero’s screw tap.

When it came to the screw-down press, however, Hero had to find a way to cut the threads inside a hole, leaving the heavy cross-beam intact. This was a challenge, but Hero was undeterred. He invented what is, as
far as we know, the world’s first screw tap. It was a box containing a wooden lead screw, guided by several
tylos.
The tip of the lead screw was fitted with an iron cutter. With the box firmly attached to the piece of wood in which a hole had been drilled, the lead screw was turned, and the cutter descended into the hole. “And we turn it till it comes into the plank, and we keep on turning it up and down, and we serve the wedge with blows again and again, until we have cut out the female screw with the furrow we wanted,” instructed Hero. “And so we have made the female screw.”
¹¹
In
1932
, a Danish historian, Aage Gerhardt Drachmann, made a drawing of the screw tap from Hero’s detailed written description. When a colleague challenged the practicability of the device and declared it “technically impossible,” the intrepid Dane built a working model and successfully threaded a two-inch hole in a beechwood plank.
¹²

There is textual evidence that the Romans used screw taps with iron as well as wood. Josephus, a Jewish historian who lived in the first century
A.D.
, writes of the temple in Jerusalem and describes eight-and-a-half-foot-long iron tie-rods that reinforced the supporting columns. “The head of each rod passed into the next by means of a cleverly made socket crafted in the form of a screw.” Later, he elaborates: “They were held by these sockets, the male fitting into the female.”
¹³
These female sockets must have been threaded. Pappus of Alexandria,
one of the last great Greek mathematicians, who lived in the fourth century, writes that “a screw is constructed having a helix with oblique threads in the drum made to fit in with
another
[emphasis added],” which might be a nut and bolt.
¹⁴
Vitruvius is clearer. In describing a
trispast,
a crane that looked like a wooden A-frame, he writes that the two timbers “are fastened together at the upper end by a bolt.”
¹⁵
Oddly, archaeological evidence for nuts and bolts is extremely slim. Indeed, there is only one surviving Roman nut. Displayed in the Provincial Museum of Bonn, it is wrought iron, approximately one and a quarter inches square with a half-inch-diameter threaded hole. The nut was discovered in the
1890
s among Roman relics dated
A.D.
180
–
260
on the site of a fortified camp in Germany.
¹⁶
No bolt was found.

If nuts and bolts were used only to assemble demountable structures such as the crane described by Vitruvius, that may explain why so few have been found. One thing is certain, the Romans—despite being skilled ironworkers and having invented nails—never made the connection between bolts and screws. Roman screws and screwdrivers are nowhere written about, and none have been discovered. “Necessity is the mother of invention” is an old Roman saying. Of course, the Romans had neither matchlocks nor butt hinges, so perhaps they felt no pressing need to develop a small, effective fastener. On the other hand, they did use bellows, and as
Agricola pointed out, screws were superior to nails. Yet there is no such thing as a technological imperative. It would take another fourteen hundred years for the screw to appear. That is, it would take another fourteen hundred years for a mechanical poet to realize that the helix that could press olives, stretch broken limbs, and adjust surveying instruments could also serve as a kind of threaded nail.

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.

H
ERO OF
A
LEXANDRIA
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.

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.