On Christmas Day in 1741, when Swedish scientist Anders Celsius first noted down the temperature in his Uppsala observatory using his own 100-point – or “Centi-grade” – scale, he would have had no idea that this was to be his greatest legacy.
A newly published, engrossing biography – Celsius: a Life and Death by Degrees – by Ian Hembrow, tells the life story of the man whose name is so well known. The book reveals the broader scope of Celsius’ scientific contributions beyond the famous centigrade scale, as well as highlighting the collaborative nature of scientific endeavours, and drawing parallels to modern scientific challenges such as climate change.
That winter, Celsius, who was at the time in his early 40s, was making repeated measurements of the period of a pendulum – the time it takes for one complete swing back and forth. He could use that to calculate a precise value for the acceleration caused by gravity, and he was expecting to find that value to be very slightly greater in Sweden than at more southern latitudes. That would provide further evidence for the flattening of the Earth at the poles, something that Celsius had already helped establish. But it required great precision in the experimental work, and Celsius was worried that the length (and therefore the period) of the pendulum would vary slightly with temperature. He had started these measurements that summer and now it was winter, which meant he had lit a fire to hopefully match the summer temperatures. But would that suffice?
Throughout his career, Celsius had been a champion of precise measurement, and he knew that temperature readings were often far from precise. He was using a thermometer sent to him by the French astronomer Joseph-Nicolas Delisle, with a design based on the expansion of mercury. That method was promising, but Delisle used a scale that took the boiling point of water and the temperature in the basement of his home in Paris as its two reference points. Celsius was unconvinced by the latter. So he made adaptations (which are still there to be seen in an Uppsala museum), twisting wire around the glass tube at the boiling and freezing points of water, and dividing the length between the two into 100 even steps.
The centigrade scale, later renamed in his honour, was born. In his first recorded readings he found the temperature in the pleasantly heated room to be a little over 80 degrees! Following Delisle’s system – perhaps noting that this would mean he had to do less work with negative numbers – he placed the boiling point at zero on his scale, and the freezing point at 100. It was some years later, after his death, that a scientific consensus flipped the scale on its head to create the version we know so well today.
Hembrow does a great job at placing this moment in the context of the time, and within the context of Celsius’ life. He spends considerable time recounting the scientist’s many other achievements and the milestones of his fascinating life.
The expedition that had established the flattening of the Earth at the poles was the culmination of a four-year grand tour that Celsius had undertaken in his early 30s. Already a professor at Uppsala University, in the town where he had grown up in an academic family, he travelled to Germany, Italy, France and London. There he saw at first hand the great observatories that he had heard of and established links with the people who had built and maintained them.
On his extended travels he became a respected figure in the world of science and so it was no surprise when he was selected to join a French expedition to the Arctic in 1736, led by mathematician Pierre Louis Maupertuis, to measure a degree of latitude. Issac Newton had died just a few years before and his ideas relating to gravitation were not yet universally accepted. If it could be shown that the distance between two lines of latitude was greater near the poles than on the equator, that would prove Newton right about the shape of the Earth, a key prediction of his theory of gravitation.
After a period of time in London equipping themselves with the precision instruments, the team started the arduous journey to the Far North. Once there they had to survey the land – a task made challenging by the thick forest and hilly territory. They selected nine mountains to climb with their heavy equipment, felling dozens of trees on each and then creating a sturdy wooden marker on each peak. This allowed them to create a network of triangles stretching north, with each point visible from the two next to it. But they also needed one straight line of known length to complete their calculations. With his local knowledge, Celsius knew that this could only be achieved on the frozen surface of the Torne river – and that it would involve several weeks of living on the ice, working largely in the dark and the intense cold, and sleeping in tents.
After months of hardship, the calculations were complete and showed that the length of one degree of latitude in the Arctic was almost 1.5 km longer than the equivalent value in France. The spheroid shape of the Earth had been established.
Of course, not everybody accepted the result. Politics and personalities got in the way. Hembrow uses this as the starting point for a polemic about aspects of modern science and climate change with which he ends his fine book. He argues that the painstaking work carried out by an international team, willing to share ideas and learn from each other, provides us with a template by which modern problems must be addressed.
Considering how often we use his name, most of us know little about Celsius. This book helps to address that deficit. It is a very enjoyable and accessible read and would appeal, I think, to anybody with an interest in the history of science.
- 2024 History Press 304pp £25hb
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