One Good Turn: A Natural History of the Screwdriver and the Screw (11 page)

The medieval printing press was likely an adaptation of a similar press that was used in paper-making. Stacks of damp sheets of paper, alternating with layers of felt cloth, were squeezed dry between two boards. But there might have been other models, for presses had many applications in medieval times. Linen presses, which were found in every large household, gave freshly woven cloth a smooth, lustrous finish. Olive and grape presses were used for pressing olive oil and wine. Apple presses made apple juice; seed presses squeezed oil from rapeseed and flax. All these presses used a vertical screw that was turned to exert downward pressure.

The printing press and the paper press were medieval devices, but linen presses had been used since Roman times. A picture of a linen press with a heavy
wooden frame and not one but two screws survives in a Pompeian fresco. The origins of olive and wine presses are ancient, too. The Roman architect Marcus Vitruvius Pollio, who lived sometime in the first century
B.C.
, mentions olive-oil presses in
The Ten Books on Architecture.
In a passage on farmhouse planning, he describes a “pressing room” where olives are pressed to make oil. He writes that the room should be not less than forty feet long in order to accommodate the traditional beam press, but adds that such a large room is not required if the press is worked by “turning screws.”
⁶

Medieval paper press.

Roman linen press, from a mural painting found in Pompeii.

The discovery of the screw press is described in detail in
Historia Naturalis,
published in
A.D.
66
by Pliny the Elder. Pliny credits Hero of Alexandria with the invention. Hero was a mathematician (he discovered the formula for calculating the area of a triangle), but like most ancient mathematicians, he was also interested in mechanics. According to Pliny, Hero began his experiments with presses by improving the traditional beam press. The beam press consisted of a long wooden beam (
prelum
) whose end was inserted into a pocket in a wall. The beam was raised, and a bag of macerated olive pulp was placed beneath it, like a nut in a giant nutcracker. Then the beam was pulled down by means of a rope wound around a drum. To get rid of the clumsy rope, Hero inserted a large wooden screw, fixed to floor and
ceiling, through the beam. As a nut was turned, the beam was forced down. This worked well, but Hero found that the nut had a tendency to jam. He then took a different tack. He attached a heavy stone to the bottom of the screw. Now, as the screw was turned, it lifted the stone, whose weight carried the beam down. “When you have hung up the stone and left it to itself,” Hero wrote, “the
prelum
will do the pressing without your having to repeat the pressure several times.”
⁷

Pliny describes the weighted beam press as “very much praised.” Nevertheless, pulling
up
a weight in order to pull
down
a beam is hardly elegant, and Hero was not satisfied. Instead of pulling a weight up, he asked himself, what if the screw was used to push down? At the same time, what if the
prelum
was eliminated altogether? Thus Hero invented the direct-screw press, the ancestor of the printing press. In fact, his machine is virtually indistinguishable from later presses. “We fix on the table two uprights,” Hero writes in his detailed and lucid description, “which carry the crosspiece. . . . The screw hole should be in the middle of the crosspiece. The screw is put through this hole and turned by means of handspikes till it reaches the lid which is laid on the
galeagra
[the box containing the fruit] and presses it down and the juice flows.”
^II
,
8

The screw-down press is a marvelous invention, not only because it is simple and compact but also because it is capable of enormous pressure. The downward force is a direct function of the ratio between the pitch of the screw and the circumference described by the handspike. For example, imagine a press of the type described by Hero whose large screw has a pitch of one inch, and which is turned by means of a handspike three feet long. If a man applies a force of, say, forty pounds to the handspike, the pressure exerted on the olive pulp will be more than nine
thousand
pounds. The ability of a single man—working without animals or waterpower—to exert this kind of force was unprecedented.

Hero’s direct-screw press.

—

Hero invented a variety of machines in which he often incorporated a common mechanical device that the ancient Greeks called an “endless screw,” today referred
to as a worm gear. A worm gear is a combination of a long screw and a toothed wheel; each revolution of the screw advances the wheel a minute distance. The mechanical advantage depends on the pitch of the screw and the number of teeth in the wheel. Hero incorporated several endless screws into his hodometer, or “road measurer.” The instrument was fixed atop a cart whose axle powered a train of endless screws that, at predetermined intervals, released a pebble into a box. The surveyor had only to count the pebbles to compute the distance traversed. Hero also invented the
dioptra,
an ancestor of the theodolite. The
dioptra
was mounted on a tripod or pedestal and the surveyor squinted down a sight whose horizontal and vertical orientation he adjusted by means of two worm gears, here used as regulating screws.

The screws used in ancient worm gears were usually bronze; screws for presses were wood. Both screws were made by tracing a helix onto a cylinder or rod and filing or carving the thread by hand. The template was a sheet of soft metal in the shape of a right-angled triangle. According to the ancient instructions, the triangle was wrapped around the rod so that one arm of the right angle was parallel to the axis, the hypotenuse automatically tracing a helix on the rod.
⁹
I couldn’t quite imagine this, so I thought I would try it using a piece of triangular paper and a broomstick handle. When I finished wrapping the triangular piece of paper around the stick, indeed, the edge of the paper made a neat helix. The problem was that there was no way to trace it without cutting through the paper. The instructions clearly stated that the template was reused. I realized that my mistake was to start wrapping with the vertical of the triangle against the broomstick. If I started with the point of the triangle, I could trace the hypotenuse, section by section, as I unwrapped the paper.

A worm gear.

Tracing a helix.

Another Greek device that used screws was called a “tortoise.” The tortoise was a primitive nut made of a block of wood, drilled with a smooth hole, inside which was an iron or copper peg, called a
tylos.
The screw went into the hole, and as the
tylos
engaged the rotating thread, the tortoise “crawled” along the rotating screw. The tortoise is said to have originated in an apparatus for resetting fractured bones and is attributed to Andreas, a physician who lived in the third century
B.C.
This machine, which has an unfortunate resemblance to the torturer’s rack, used the tortoise to gradually pull a harness that stretched the fractured limb. The tortoise was also used in adjustable obstetric instruments such as clamps and dilators.
¹⁰

Since the friction between the
tylos
and the threads of the screw was too great to allow much force to be exerted, the tortoise could be used only with relatively small devices. The massive screw of Hero’s beam press
would have jammed if he had used a
tylos.
He needed a different way of engaging the screw. Pondering the problem, Hero made another momentous discovery: the (male) screw has a (female) counterpart: the nut. We do not know exactly how he made this breakthrough. Perhaps he tried several pegs and intuited a continuous female thread. Maybe he thought it through mathematically. Or did it come in a flash of inspiration? Once he had the idea. It was a relatively simple matter to make a nut: using a Roman auger, he drilled a hole in a block of wood, split it in two, carved the female threads, and put the parts back together.