May 192017


On this date in 1743 the Lyonnais physicist Jean-Pierre Christin, permanent secretary of the Académie des sciences, belles-lettres et arts de Lyon, working independently of the Swedish astronomer Anders Celsius (who had developed a similar (but inverted) scale), published the design of a mercury thermometer, the “Thermometer of Lyon” built by the craftsman Pierre Casati that used a scale where zero represented the freezing point of water and 100 represented the boiling point of water at one standard atmosphere. It was, and still is (sometimes), called the centigrade scale although more usually it is called the Celsius scale to honor the first creator even though his scale is not quite the same as the centigrade scale. I tend to vacillate between the two names because I grew up calling it centigrade which seems more etymologically satisfying to me – “centi” (100), “gradus” (degree). Honoring people is all right too, though, as for many SI units: joule, amp, volt, etc. etc. I’ll dribble on a bit about the history of the Celsius scale and then turn my attention to why the US is so resistant to the metric system when the rest of the world uses it more or less happily – even Britain, where such changes do not come easily.


As it happens, the Fahrenheit scale, developed by the Dutch-German-Polish physicist, inventor, and scientific instrument maker Daniel Gabriel Fahrenheit, was published only about 20 years before Celsius and Christin published details of their scales, because at that time there was a pressing need in science for accurate measurements of temperature. Fahrenheit’s scale had three calibration points: the freezing point of a stable mix of ice, water, and ammonium chloride  (0°F), the freezing point of distilled water (32°F), and mean human body temperature (96°F). The latter reference point was later shifted slightly higher. The boiling point of distilled water at one atmosphere was set at 212°F, making the range between the freezing and boiling points of water 180 degrees. 180 is a highly composite number (or anti-prime), meaning that it has numerous divisors, so that, in theory, the scale is useful for mathematical calculations that will result in whole number solutions to various equations.


In 1742 Anders Celsius (1701–1744) published details of  a temperature scale which was the reverse of the scale now known by his name: 0 represented the boiling point of water, while 100 represented the freezing point of water. In his paper “Observations of two persistent degrees on a thermometer,” he documented his experiments showing that the melting point of ice is essentially unaffected by pressure. He also determined with remarkable precision how the boiling point of water varied as a function of atmospheric pressure. He proposed that the zero point of his temperature scale, being the boiling point, would be calibrated at the mean barometric pressure at mean sea level.

In 1743 Jean-Pierre Christin published his work on a centigrade scale, and in 1744, coincident with the death of  Celsius, Swedish botanist Carolus Linnaeus (1707–1778) reversed Celsius’ scale, but otherwise kept the intervals between degrees the same. Here we have a, not very well known, example of a common habit of scientists coming up with the same results independently. In this case the coincidence is undoubtedly due to the fact that a metric system of measures across the board makes a great deal of sense for computational purposes. Time is the one variable that won’t play nice.

Linnaeus’ custom-made “linnaeus-thermometer,” for use in his greenhouses, was made by Daniel Ekström, Sweden’s leading maker of scientific instruments at the time and whose workshop was located in the basement of the Stockholm observatory. As it happens, numerous physicists, scientists, and instrument makers at the time are credited with having independently (or semi-independently) developed a centigrade scale; among them, Pehr Elvius, the secretary of the Royal Swedish Academy of Sciences (which had an instrument workshop) and with whom Linnaeus had been corresponding; Daniel Ekström, the instrument maker; and Mårten Strömer (1707–1770) who had studied astronomy under Anders Celsius.

The first known Swedish document reporting temperatures in the centigrade scale is the paper “Hortus Upsaliensis” dated 16 December 1745 that Linnaeus wrote for a student of his, Samuel Nauclér. In it, Linnaeus recounted the temperatures inside the orangery at the University of Uppsala Botanical Garden:

…since the caldarium (the hot part of the greenhouse) by the angle of the windows, merely from the rays of the sun, obtains such heat that the thermometer often reaches 30 degrees, although the keen gardener usually takes care not to let it rise to more than 20 to 25 degrees, and in winter not under 15 degrees…

For a long time the Fahrenheit and centigrade systems had roughly equal followings. In Australia where I grew up, in England where I finished secondary school and attended university, and the US where I lived for 35 years, Fahrenheit ruled in weather reporting and (mostly) in the laboratory. Nowadays only the US and a few scattered islands in the Atlantic and Pacific (mostly US dependencies) use Fahrenheit for weather reporting, and every country in the world uses centigrade for scientific purposes (or the closely related Kelvin scale). Here is a map of the world with those countries using centigrade colored in grey, and those using Fahrenheit colored in green. (You will have to click on the map to enlarge it to see the tiny islands).

Why is the US so resistant to conversion to the metric system in general? I’d say that there are multiple reasons, including a degree of mindless conservatism (coupled with a resistance to the cost of changing).  Clearly such resistance has its down side. For example, the Hubble telescope had to be retrofitted at great expense and inconvenience because the US engineers who designed and built it confused metric and Imperial units of measurement, and ended up at the outset with a telescope in space with a prime reflecting mirror that could not be focused properly.

When it comes to weather reporting I understand why people in the US don’t want to switch from Fahrenheit to centigrade. Fahrenheit has a human dimension to it that centigrade lacks. I can recalibrate Fahrenheit, used for weather reporting, to what I will call the Juan Alejandro Bloody-Bloody Scale thus: 0°F is BLOODY COLD and 100°F is BLOODY HOT. Both ends of this scale represent well understood extremes with round decimal numbers. Centigrade is not round at all at those temperatures. 0°F  is -17.7778°C and 100°F is 37.778°C (approximately).  For round numbers in the centigrade scale you need to pick 0 (which is critical in some ways, such as for frost or plant growth, but not especially cold for humans), and 40 (which is insanely hot, and not very common). Most places I have lived in the world regularly experience one or other of the extremes of the Fahrenheit scales, but not both. My home in the Catskills in New York, however, had the good fortune to experience both on a regular basis. Not the prime reason, but one of several reasons that I do not live there any more.

What I am getting at is that both 0 and 100 in the Fahrenheit scale represent significant milestones (or turning points) in the ways humans feel about local weather conditions. When someone says, “It’s going to hit one hundred (or zero) today” there’s a sense of importance derived from the number itself. There’s a recognizability to the number even though the difference between 99°F and 100°F is hardly noticeable. Hundreds mark significant achievements in human terms: 100 years old, 100th anniversary (i.e. centennial) etc. So in that sense the Fahrenheit scale has a more human feel to it than centigrade (in my opinion). Even though Fahrenheit is not intrinsically decimal, it has a decimal feel to it where it counts in human experience.

Thermometers have limited, but very important, uses in cooking. In particular they are invaluable in sugar cookery. If you check out the HINTS tab of this blog you will find my notes on the various stages of sugar cooking for different confections and the temperatures needed to achieve those stages (in centigrade). For a recipe I want to turn to Lyon, home of Jean-Pierre Christin whom we are celebrating today, and it would be great if there were a local recipe that I could share that uses a sugar thermometer. Lyon is certainly a major culinary center and there are numerous candied treasures to sample, such as the legendary pink pralines or coussins de Lyon. But . . . their production involves trade secrets. Sorry, save your pennies for the air fare. The best I can offer is a recipe for marrons glacés (candied chestnuts) which are a Lyon specialty. Even there I recommend going to Lyon rather than attempting to make them yourself. It’s a very fiddly and time-consuming job.

Let’s start with the French name and what it implies. There are two French words for “chestnut” – marron and châtaigne. The châtaigne is a low grade chestnut normally used for roasting and the typical chestnut that you find in stores outside of southern France and northern Italy. The marron is a high quality chestnut that can cost four or five times more than regular chestnuts, and are the ones you need for this recipe. One thing that is simple about this recipe is the ratio of ingredients 1:1:1:1 – 1 part peeled chestnuts to 1 part sugar to 1 part water to 1 vanilla bean. Now it gets demanding.

Take each chestnut and cut through the tough outer skin all the way around the chestnut so that you cut to the lower membrane, but do not pierce the meat. With a little labor you can peel off the outer skin, but it goes quicker if you place the chestnuts in the microwave on high heat for 20 seconds. This produces some steam as the chestnuts cook a little, loosening the skin. Peel off the tough outer skin being careful not to damage the meat. Then remove the inner skin. Some people use the point of a paring knife, others scrub off the skin with steel wool or an abrasive pad (used only for cooking). Using an abrasive rather than a knife makes damage to the meat less likely. Be prepared for a certain number of damaged chestnuts. These will not make pretty confections, but candy them anyway and then chop them for use with ice cream or in pies and cakes.

Place the water, sugar, and vanilla bean (split lengthwise) in a saucepan and bring to a boil, stirring to dissolve the sugar. Place the peeled chestnuts in a wire basket and lower them into the syrup. Boil vigorously for 1 minute.  Take from the heat and let cool. Keep the chestnuts 24 hours in the syrup, then repeat the process of boiling, cooling, and preserving for 24, hours three times. After the last cooling remove the wire basket from the syrup and separate the chestnuts on wire racks to dry. They are best if eaten quickly !!

Nov 272015


Today is the birthday (1701) of Anders Celsius, a Swedish astronomer, physicist and mathematician. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories in Germany, Italy and France. He founded the Uppsala Astronomical Observatory in 1741, and in 1742 proposed a temperature scale which now bears his name.

Celsius was born in Uppsala in Sweden, but his family originated from Ovanåker in the province of Hälsingland. Their family estate was at Doma, also known as Höjen or Högen (locally as Högen 2). The name Celsius is a latinization of the estate’s name (Latin celsus “mound”).

As the son of an astronomy professor, Nils Celsius, and the grandson of the mathematician Magnus Celsius and the astronomer Anders Spole, Celsius chose a career in science. He was a talented mathematician from an early age. Anders Celsius studied at Uppsala University, where his father was a teacher, and in 1730 he too, became a professor of astronomy there.


In 1730, Celsius published the Nova Methodus distantiam solis a terra determinandi (New Method for Determining the Distance from the Earth to the Sun). His research also involved the study of auroral phenomena, which he conducted with his assistant Olof Hiorter, and he was the first to suggest a connection between the aurora borealis and changes in the magnetic field of the Earth. He observed the variations of a compass needle and found that larger deflections correlated with stronger auroral activity. At Nuremberg in 1733, he published a collection of 316 observations of the aurora borealis made by himself and others over the period 1716–1732.

Celsius traveled frequently in the early 1730s, including to Germany, Italy and France, when he visited most of the major European observatories. In Paris he advocated the measurement of an arc of the meridian in Lapland. In 1736, he participated in the expedition organized for that purpose by the French Academy of Sciences, led by the French mathematician Pierre Louis Maupertuis (1698–1759) to measure a degree of latitude. The aim of the expedition was to measure the length of a degree along a meridian, close to the pole, and compare the result with a similar expedition to Peru, near the equator. The expeditions confirmed Isaac Newton’s belief that the shape of the earth is an ellipsoid flattened at the poles.

In 1738, he published the De observationibus pro figura telluris determinanda (Observations on Determining the Shape of the Earth). Celsius’ participation in the Lapland expedition won him much respect in Sweden with the government and his peers, and played a key role in generating interest from the Swedish authorities in donating the resources required to construct a new modern observatory in Uppsala. He was successful in the request, and Celsius founded the Uppsala Astronomical Observatory in 1741. The observatory was equipped with instruments purchased during his long voyage abroad, comprising the most modern instrumental technology of the period.

In astronomy, Celsius began a series of observations using colored glass plates to record the magnitude (a measure of brightness) of certain stars. This was the first attempt to measure the intensity of starlight with a tool other than the human eye. He made observations of eclipses and various astronomical objects and published catalogs of carefully determined magnitudes for some 300 stars using his own photometric system (mean error=0.4 mag).


Celsius was the first to perform and publish careful experiments aiming at the definition of an international temperature scale on scientific grounds. In his Swedish paper “Observations of two persistent degrees on a thermometer” he reports on experiments to check that the freezing point is independent of latitude (and of atmospheric pressure). He determined the dependence of the boiling of water on atmospheric pressure which was accurate even by modern day standards. He further gave a rule for the determination of the boiling point if the barometric pressure deviates from a certain standard pressure. He proposed the Celsius temperature scale in a paper to the Royal Society of Sciences in Uppsala, the oldest Swedish scientific society, founded in 1710. His thermometer was calibrated with a value of 100° for the freezing point of water and 0° for the boiling point. In 1745, a year after Celsius’ death, the scale was reversed by Carl Linnaeus to facilitate more practical measurement. Celsius originally called his scale “centigrade” derived from the Latin for “hundred steps”. For years it was simply referred to as the Swedish thermometer.


Celsius conducted many geographical measurements for the Swedish General map, and was one of earliest to note that much of Scandinavia is slowly rising above sea level, a continuous process which has been occurring since the melting of the ice from the latest ice age. However, he wrongly posited the notion that the water was evaporating.


In 1725 he became secretary of the Royal Society of Sciences in Uppsala, and served at this post until his death from tuberculosis in 1744.

Here’s a map of the world showing all the nations that use the Celsius scale, and those that use the Fahrenheit scale. Hmmmm.


Barely visible are the Bahamas, Belize, the Cayman Islands, and the Republic of Palau.

This is why I often use both Celsius and Fahrenheit in my recipes (sop to the USA). Anyway, I’m slightly haphazard about metric versus imperial measure in general because I’m not a big fan of precision in cooking in general (except baking). It’s hard enough for me to include measures at all. With oven temperatures I give exact measures because ovens come that way, but I don’t think in those terms. I think in heuristic terms, such as hot, medium, etc. Partly this is because ovens are so variable. At one time I used an internal oven thermometer, but these days I wing it. My oven in China never got hot enough for me, and my current one seems to have two settings – furnace and off. I manage.

The cloudberry (Rubus chamaemorus) is native to Sweden and very popular there. Sadly, it is very difficult to cultivate, so it’s almost impossible to find fresh cloudberries outside of northern latitudes. Nonetheless, I am going to give you a recipe for cloudberry ice cream made with fresh berries. Slightly modified, this recipe can be made with cloudberry preserves, which are much more easily found worldwide. I am choosing ice cream for today’s celebration because Celsius determined that the freezing point of pure water was invariant (and so became one end of his temperature scale). Also, Swedes are the heaviest consumers of ice cream in the world. Cloudberry ice cream and chilled cloudberry cream are common favorites.


Hjortronglass (Cloudberry Ice Cream)


18 ounces fresh cloudberries (about 4 cups)
¾ cup sugar, divided
4 large egg yolks
kosher salt
1½ cups heavy cream, divided
2 tsp fresh lemon juice


Cook the berries and ¼ cup of sugar in a medium saucepan over medium heat until the berries are soft and starting to release their juices. Increase the heat to medium-high and bring to a boil. Boil, stirring occasionally to prevent sticking, until the mixture thickens slightly, about 5 minutes. Set aside ½ cup of sauce. Purée the remaining sauce in a blender until smooth, and strain through a fine-mesh sieve into a measuring glass (you should have about 1 cup). Let cool.

Whisk the egg yolks, a pinch of salt, and the remaining ½ cup sugar in a medium bowl until lightened in color. Bring 1 cup of cream to a boil in a medium saucepan. Immediately remove from the heat and very gradually whisk half of the cream into the egg yolk mixture. Be very careful here because you can easily scramble the egg. Whisking constantly, add the egg mixture to the remaining cream in the pan and then cook over medium heat, stirring constantly, until the custard is thickened, about 2 minutes. Strain through a fine-mesh sieve into a large bowl. Chill until cold.

Whisk the custard, berry purée, lemon juice, and remaining ½ cup of cream until smooth. Process in an ice cream maker of your choice.

Spoon in the reserved berry sauce, then scrape the ice cream into an airtight container (you want nice streaks of sauce still visible). Cover and freeze until firm, at least 2 hours.