Weather wise

IN THE WHOLE of Aristotle's long work on meteorology (in the translation that I keep, anachronistically, on my lap-top computer), he uses the word "temperature" only twice. Curiously, although the ancient Greeks understood a good deal about the physics of heat, they never seemed to think of devising a scale for measuring it.

The invention of the thermometer is usually credited to Galileo, though his heat-measuring device was designed to detect only whether temperature was rising or falling, not by how much. It was Galileo's friend, the physician Sanctorio Sanctorio (so good, they named him twice) who was probably the first to add a scale to the instrument and thus come up with the earliest true thermometer. The apparatus used by both men was a thin-necked glass vessel inverted in a bowl of liquid. As the air in the vessel was heated or cooled, it expanded or contracted, and the water level in its neck rose and fell. Sanctorio (1561-1636) tried heating the vessel with a candle, then freezing it with snow, noting the levels in each case, then dividing the interval between them into 110 equal parts. Sanctorio, incidentally, also invented a syringe for removing bladder stones and a pendulum for measuring pulse rates.

The next unlikely hero of thermometry was the Grand Duke Ferdinand II of Tuscany, who, around 1644, had the grand idea of sealing the end of a liquid-filled tube with a bulb. This, and later developments, were made possible by the skill of the Florentine glass-blowers.

The 18th century was a time of competition among rival scales of temperature. Of these, three won out over their rivals.

The gold medal must go to the Swiss astronomer Anders Celsius (1701-44). He was one of the first to demonstrate the flattening of the Earth at its poles, but now is remembered for his invention of the Celsius scale of temperature - though his version of it was the reverse of the one we use today. The original Celsius scale had zero as the boiling-point of water and 100 as its freezing-point.

Our silver medallist is the Polish-born Dutch instrument maker Gabriel Daniel Fahrenheit (1686-1736), who wanted a scale that extended below freezing and above boiling without having to descend into negative numbers. He chose as his zero the freezing point of salt water. For its other fixed point, he designated 96 as the temperature of the human body (suggesting that he was chilly at the time). He then measured the boiling point of water to be 212.

A long way behind, in third place, comes Rene Antoine Ferchault de Reaumur (1683-1757) whose 80-point scale between freezing and boiling water never quite attained the eminence of the other two. But he did write a splendid six-volume history of insects.