CHANGES IN THE ATMOSPHERE
Concerning how a toy built in Alexandria failed to inspire, and how a glass tube made in italy succeeded; the spectacle of two german hemispheres attached to sixteen german horses; and the critical importance of nothing at all to get to crofton from Birmingham, you take the M5 south about sixty miles to Brockworth and then change to the A417, which meanders first east, then southwest, then southeast, for another forty-six miles, changing, for no apparent reason, into the A419, and then the A436. In Burbage, you turn left at the Wolfhall Road and follow it another mile, across the railroad tracks and over the canal. The reason for making this three-hour journey (not counting time for wrong turns) is visible for the last quarter-mile or so: two red brick buildings next to a sixty-foot-tall chimney.
The Crofton Pump Station in Wiltshire contains the oldest steam engine in the world still doing the job for which it was designed. Every weekend, its piston-operated beam pumps twelve tons of water a minute into six eight-foot-high locks along the hundred-mile-long Kennet and Avon Canal. The engine itself, number 42B-the figure "B. 42" is still visible on the engine beam-is so called because it was the second engine with a forty-two-inch cylinder produced by the Birmingham manufacturer Boulton & Watt. It was entered in the company's order book on January 11, 1810, and installed almost precisely two years later. Except for a brief time in the 1960s, it has run continuously ever since.
First encounters with steam power are usually unexpected, inadvertent, and explosive; the cap flying off a defective teakettle, for example. No surprise there; the expansive property of water when heated past a certain point was known for thousands of years before that point was ever measured, and to this day it's what drives the turbine that generates most of our electricity, including that used to power the light by which you are reading this book. The relationship between the steam power of a modern turbine and the kind used to pump the water out of the Kennet and Avon Canal is, however, anything but direct. By comparison, the mechanism of engine 42B is a thing of Rube Goldberg-like complexity, with levers, cylinders, and pistons yoked together by a dozen different linkages, connecting rods, gears, cranks, and cams, all of them moving in a terrifyingly complicated dance that is at once fascinating, and eerily quiet- enough to occupy the mechanically inclined visitor, literally, for hours. When the engine is "in steam," it somehow causes the twenty- six-foot-long cast iron beams to move, in the words of Charles Dickens, "monotonously up and down, like the head of an elephant in melancholy madness."
There is, however, something odd about the beams, or rather about the pistons to which they are attached. The pistons aren't just being driven up by the steam below them. The power stroke is also down: toward the steam chamber. Something is sucking the pistons downward. Or, more accurately, nothing is: a vacuum.
Using steam to create vacuum was not the sort of insight that came an instant after watching a teakettle lid go flying. It depended, instead, on a journey of discovery and diffusion that took more than sixteen centuries. By all accounts the trip began sometime in the first century ce, on the west side of the Nile Delta, in the Egyptian city of Alexandria, at the Mouseion, the great university at which first Euclid and then Archimedes studied, and where, sometime around 60 CE, another great mathematician lived and worked, one whose name is virtually always the first associated with the steam engine: Heron of Alexandria.
The Encyclopaedia Britannica entry for Heron-occasionally, Hero-is somewhat scant on birth and death dates; as is often the case with figures from an age less concerned with such trivia, it uses the abbreviation "fl." for the latin floruit, or "flourished." And flourish he did. Heron's text on geometry, written sometime in the first century but not rediscovered until the end of the nineteenth, is known as the Metrika, and includes both the formula for calculating the area of a triangle and a method for extracting square roots. He was even better known as the inventor of a hydraulic fountain, a puppet theater using automata, a wind-powered organ, and, most relevantly for engine 42B, the aeolipile, a reaction engine that consisted of a hollow sphere with two elbow-shaped tubes attached on opposite ends, mounted on an axle connected to a tube suspended over a cauldron of water. As the water boiled, steam rose through the pipe into the sphere and escaped through the tubes, causing the sphere to rotate.
Throughout most of human history, successful inventors, unless wealthy enough to retain their amateur status, have depended on patronage, which they secured either by entertaining their betters or glorifying them (sometimes both). Heron was firmly in the first camp, and by all accounts, the aeolipile was regarded as a wonder by the wealthier classes of Alexandria, which was then one of the richest and most sophisticated cities in the world. Despite the importance it is given in some scientific histories, though, its real impact was nil. No other steam engines were inspired by it, and its significance is therefore a reminder of how quickly inventions can vanish when they are produced for a society's toy department.
In fact, because the aeolipile depended only upon the expansive force of steam, it should probably be remembered as the first in a line of engineering dead ends. But if the inspirational value of Heron's steam turbine was less than generally realized, that of his writings was incomparably greater. He wrote at least seven complete books, including Metrika, collecting his innovations in geometry, and Automata, which described a number of self-regulating machines, including an ingenious mechanical door opener. Most significant of all was Pneumatika, less for its descriptions of the inventions of this remarkable man (in addition to the aeolipile, the book included "Temple Doors Opened by Fire on an Altar," "A Fountain Which Trickles by the Action of the Sun's Rays," and "A Trumpet, in the Hands of an Automaton, Sounded by Compressed Air," a catalog that reinforces the picture of Heron as antiquity's best toymaker) than for a single insight: that the phenomenon observed when sucking the air out of a chamber is nothing more than the pressure of the air around that chamber. It was a revelation that turned out to be utterly critical in the creation of the world's first steam engines, and therefore of the Industrial Revolution that those engines powered.
The idea wasn't, of course, completely original to Heron; the idea that air is a source of energy is immeasurably older than science, or even technology. Ctesibos, an inventor and engineer born in Alexandria three centuries before Heron, supposedly used compressed air to operate his "water organ" that used water as a piston to force air through different tubes, making music.
Just as the ancients realized that moving air exerts pressure, they also recognized that its absence did something similar. The realization that sucking air out of a closed chamber creates a vacuum seems fairly obvious to any child who has ever placed a finger on top of a straw-as indeed it was to Heron. In the preface to Pneumatika, he wrote, if a light vessel with a narrow mouth be taken and applied to the lips, and the air be sucked out and discharged, the vessel will be suspended from the lips, the vacuum drawing the flesh towards it that the exhausted space may he filled. It is manifest from this that there was a continuous vacuum in the vessel...thus producing what a modern scholar has called a "very satisfactory theory of elastic fluids."
Satisfactory to a twenty-first-century child, and a first-century mathematician, but not, unfortunately, for a whole lot of people in between. To them, the idea that space could exist absent any occupants, which seems self-evident, was evidently not, and the reason was the dead hand of the philosopher-scientist who tutored Alexandria's founder. Aristotle argued against the existence of a vacuum with unerring, though curiously inelegant, logic. His primary argument ran something like this:
1. If empty space can be measured, then it must have dimension.
2. If it has dimension, then it must be a body (this is something of a tautology: by Aristotelian definition, bodies are things that have dimension).
3. Therefore, anything moving into such a previously empty space would be occupying the same space simultaneously, and two bodies cannot do so.
More persuasive was the argument that a void is "unnecessary," that since the fundamental character of an object consists of those measurable dimensions, then a void with the same dimensions as the cup, or horse, or ship occupying it is no different from the object. One, therefore, is redundant, and since the object cannot be superfluous, the void must be.
It takes millennia to recover from that sort of unassailable logic, temptingly similar to that used in Monty Python and the Holy Grail to demonstrate that if a woman weighs as much as a duck, she is a witch. Aristotle's blind spot regarding the existence of a void would be inherited by a hundred generations of his adherents. Those who read the work of Heron did so through an Aristotelian scrim on which was printed, in metaphorical letters twenty feet high: NATURE ABHORS A VACUUM.
Given that, it is something of a small miracle that Pneumatika, and its description of vacuum, survived at all. But survive it did, like so many of the great works of antiquity, in an Arabic translation, until around the thirteenth century, when it first appeared in Latin. And it was another three hundred years until a really influential translation arrived, an Italian edition translated by Giovanni Batista Aleotti d'Argenta and published in 1589. Aleotti's work, and subsequent translations of his translation into German, English, and French (plus five more in Italian alone), demonstrate both the demand for and availability of the book. Aleotti, an architect and engineer, was practical enough; in his annotations to his translation of the Pneumatika, he mentions the difficulty of removing a ramrod from a cannon with its touchhole covered because of the pressure of air against the vacuum therefore created-a phenomenon that could only exist if air were compressible and vacuum possible. It is testimony to the weight of formal logic that even with the evidence in front of his nose, Aleotti was still intellectually unable to deny his Aristotle.
If Aleotti was unaware of the implications of Heron's observations, he was indefatigable in promoting them, and by the seventeenth century, it can, with a wink, be said that Pneumatika was very much in the air, in large part because of the Renaissance enthusiasm for duplicating natural phenomena by mechanical means, the era's reflexive admiration for the achievements of Greek antiquity. The scientist and philosopher Blaise Pascal (who modeled his calculator, the Pascaline, on an invention of Heron's) mentioned it in D'esprit géometrique, as did the Oxford scholar Robert Burton in his masterpiece, Anatomy of Melancholy: "What is so intricate, and pleasing as to peruse...Hero Alexandrinus' work on the air engine." But nowhere was Aleotti's translation more popular than the city-state of Firenze, or Florence.
Florence, in the year 1641, had been essentially the private fief of the Medici family for two centuries. The city, ground zero for both the Renaissance and the Scientific Revolution, was also where Galileo Galilei had chosen to live out the sentence imposed by the Inquisition for his heretical writings that argued that the earth revolved around the sun. Galileo was seventy years old and living in a villa in Arcetri, in the hills above the city, when he read a book on the physics of movement titled De motu (sometimes Trattato del Moto) and summoned its author, Evangelista Torricelli, a mathematician then living in Rome. Torricelli, whose admiration for Galileo was practically without limit, decamped in time not only to spend the last three months of the great man's life at his side, but to succeed him as professor of mathematics at the Florentine Academy. There he would make a number of important contributions to both the calculus and fluid mechanics. In 1643, he discovered a core truth in the behavior of liquids in motion, known as Torricelli's theorem, that is still used to calculate the speed of a fluid when it exits the vessel that contains it. He made fundamental contributions to the development of the calculus, and to the geometry of the cycloid (the path described by a point on a rolling wheel). Less typically, he embarked on a series of investigations whose results were, literally, revolutionary.
In those investigations, Torricelli used a tool even more powerful than his well-cultivated talent for mathematical logic: He did experiments. At the behest of one of his patrons, the Grand Duke of Tuscany, whose engineers were unable to build a sufficiently powerful pump, Torricelli designed a series of apparatuses to test the limits of the action of contemporary water pumps. In spring of 1644, Torricelli filled a narrow, four-foot-long glass tube with mercury-a far heavier fluid than water-inverted it in a basin of mercury, sealing the tube's top, and documented that while the mercury did not pour out, it did leave a space at the closed top of the tube. He reasoned that since nothing could have slipped past the mercury in the tube, what occupied the top of the tube must, therefore, be nothing: a vacuum.
Even more brilliantly, Torricelli reasoned, and then demonstrated, that the amount of space at the top of the tube varied at different times of the day and month. The only explanation that accounted for his observations was that the variance was caused by the pressure of air; the more pressure on the open reservoir of mercury at the base of the tube, the higher the mercury rose within. Torricelli had not only invented, more or less accidentally, the first barometer; he had demonstrated the existence of air pressure, writing to his colleague Michelangelo Ricci, "I have already called attention to certain philosophical experiments that are in progress...relating to vacuum, designed not just to make a vacuum but to make an instrument which will exhibit changes in the atmosphere...we live submerged at the bottom of an ocean of air..."
Torricelli was not, even by the standards of his day, a terribly ambitious inventor. When faced with hostility from religious authorities and other traditionalists who believed, correctly, that his discovery was a direct shot at the Aristotelian world, he happily returned to his beloved cycloids, the latest traveler to find himself on the wrong side of the boundary line between science and technology.
--Ce texte fait référence à une édition épuisée ou non disponible de ce titre.