Bench Degree·FLUID POWERchapter

On the Cover

How the cover of every Bench Degree volume is chosen. One question, and it is not who got there first or who is famous. If we could bring anyone back to teach you this subject, who would you actually want at the bench? That is the whole test. It picks people who did the work with their hands, who checked things themselves, and who would be worth standing next to. Every one of them actually lived, which is why a cover portrait in this series is either a real document or openly a drawing, and never a photograph of something that did not happen.

Blaise Pascal, 1623 to 1662, dead at thirty-nine.

He is here for one sentence.

Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and to the walls of the container, and it acts at right angles to those walls.

That is the whole of this book. Every chapter is a consequence of it: the two syringes in Chapter 1, the bottle jack in Chapter 5, the excavator boom in Chapter 16, and the fact that your foot can stop two tons in Chapter 18. A subject with three hundred pages of consequences and one sentence of theory is unusual, and it is why the man who wrote the sentence is on the cover rather than any of the people who built the machines.

He wrote it in a treatise usually dated to around 1653, Traité de l’équilibre des liqueurs et de la pesanteur de la masse de l’air, and it was published in 1663, the year after he died. So the dates in this book’s Chapter 2 need a caveat: 1653 is when it was written, 1663 is when it appeared, and sources reasonably disagree about which to quote.

And he did not stop at the law. In the same treatise he describes what it implies: a vessel full of water with two openings, one a hundred times the area of the other, in which one man can push as hard as a hundred. He says in as many words that this is a new kind of machine for multiplying force. The law and its application were published together, and then nothing was built for a century and a half, because nobody could seal it. Chapter 2 is about that.

Why him and not Simon Stevin, and not Joseph Bramah.

Stevin, in Bruges, had the essentials of hydrostatics before 1600, including the result that pressure at the bottom of a vessel depends on the depth of the fluid and not on the shape of the container. It is sometimes called Stevin’s law for that reason, and the attribution is fair.

Bramah built the machine. He patented a working hydraulic press in 1795 and it worked because of a leather cup seal made in his workshop, and he is very nearly unknown. He was the alternative candidate for this cover and there is a good case for him.

There is no series rule that settles this, and it is worth saying so rather than inventing one. The nuclear volume put Meitner ahead of Fermi, which favours the person who explained the thing over the person who built it. The wind volume did the opposite and put la Cour ahead of Betz, because a man who built the machines and then founded a school to teach farmers to build them is a better teacher than the man who found the ceiling. The test in every case is the same one and it is not a formula: who would you want teaching you this, if you could bring back anyone. Pascal’s account is the one that got read, the one that connected the principle to a machine, and the one that made the machine inevitable once the materials existed. So Bramah gets Chapter 2 to himself instead, which in a book of this kind is arguably the better prize.

One thing about Pascal that should be told with a caveat, because this series prefers an honest gap to a good story. He is often said to have burst a strong barrel by fitting a long thin vertical tube to its lid and pouring in a few cups of water. It is a perfect demonstration and it appears in a great many books. The primary evidence that he actually performed it is thin. Treat it as an illustration rather than as history.

The rest of him, briefly, because it is absurd.

At sixteen he proved the theorem in projective geometry that still carries his name.

At nineteen, in 1642, he built the Pascaline, a mechanical calculating machine of gears and wheels that added and subtracted by carrying digits automatically. He built it to help his father, a tax official in Rouen, with columns of figures. Nine or so survive.

In 1654 he and Pierre de Fermat exchanged a series of letters about a gambling problem, and in doing so founded the mathematical theory of probability.

He wrote the Provincial Letters and the Pensées, which are read by people who have never heard of hydraulics.

He worked on the vacuum, against the prevailing view that it could not exist, and the Puy de Dôme barometer experiment of 1648, carried out at his direction by his brother-in-law, is one of the cleanest experimental results of the century.

And in the last year of his life, in 1662, he set up the carrosses à cinq sols in Paris: horse-drawn carriages running fixed routes, to a timetable, at a fixed fare, for anybody who turned up. The world’s first scheduled public transport service. He died a few months later.

Then there is the thing that makes him unavoidable in a book like this one. The SI unit of pressure is the pascal. Every kilopascal and every megapascal in these three hundred pages carries his name, which means that a reader who never learns who he was still uses him several times a page.

About the portrait, and what is honestly known about it.

There is no undisputed portrait of Pascal made from life. This matters, because the rule for this series is that a cover portrait is a document or it is openly a drawing, and it is never an image of something that did not happen.

What exists, and its standing:

A chalk drawing traditionally attributed to Jean Domat, a jurist and a friend of Pascal’s, and the image with the best claim to have been made from the living man or very close to him. It is a drawing, and it is presented here as one.

A painting long attributed to Philippe de Champaigne, which is the image most reproduced. The attribution to Champaigne is now generally doubted, and museum catalogues commonly qualify it as “attributed to” or “after”. Whether it is a likeness at all is not certain.

An engraving by Gérard Edelinck, made in 1691, twenty-nine years after Pascal’s death. It is a fine engraving and it cannot be a portrait from life; it was worked up from an earlier image.

And a death mask, which is a document of a kind, and is not a portrait.

So the cover of this volume is a drawing or an engraving, reproduced as a drawing or an engraving, with what is and is not known about its origin stated here rather than tidied away. It is not presented as a photograph, because there is no photograph to be had of a man who died in 1662, and it is not smoothed, colourised or completed.

The temptation that was refused. A generative model would produce, in a few seconds, a convincing photorealistic portrait of Blaise Pascal: correct period, correct lighting, plausible face. It would be a fabricated image of a real person, and it would look more real than the genuine one. That is precisely why it is not here. A book that spends Chapter 23 insisting on the difference between a specification and a measurement cannot open with an invented face.

A drawing loses information and admits it. A generated portrait adds information that never existed and hides that it did.

The full account of how this book was made, including which parts of it were written with the help of a language model, is on the copyright page and on the back cover, itemised using the five credits published at bookcredits.org.

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