## Marking the mercury

Welcome to Collected Wisdom, the column that's so hot it's off the chart, no matter how you measure it.

THE QUESTION: Why, on the Fahrenheit scale, does water freeze at 32 degrees and boil at 212? Chuck Mercer of Napanee, Ont., wants to know.

THE ANSWER: "The Fahrenheit scale was devised by Daniel Fahrenheit in 1724," says John D. Bailes of Toronto, president of the Canadian Metric Association. The challenge at the time, he says, was to make a dependable and easily duplicated thermometer.

David Brown, emeritus professor of physics at McMaster University in Hamilton, says Fahrenheit described his thermometer in a paper (written in Latin) published in the Philosophical Transactions of the Royal Society of London (1724-25).

"He describes a mercury-in-glass thermometer similar to the ones we use today, but he needed to mark this with a scale that could be used for recording the daily temperatures at his home in Holland. Since the sea never froze there, he chose the freezing point of sea water as his zero so that all his temperatures would be positive."

Dr. Brown says Fahrenheit marked this point on his thermometer. He next marked the freezing point of fresh water. The third mark he made was human body temperature (and he described all the methods we currently use for making this measurement). "He noticed that the distance between the second and third marks was just twice the distance between the first and second marks, so he added a third mark to divide his scale into three equal intervals."

This scale was too coarse for practical use so he divided each of these three intervals into 32 divisions, Dr. Brown says. "The resulting scale gave freezing sea water as 0 degrees, freezing fresh water as 32 degrees and body temperature as 96 degrees (not exactly correct, but close enough for his purposes)."

Using this scale, if CW might add a final word, water boils at 212 degrees.

THE QUESTION: In Canada, precipitation is reported and recorded differently - rain in millimetres and snow in centimetres, writes Sandy MacDonald of Toronto. Why?

THE ANSWER: Scientists try to use measurements that are both accurate and meaningful, writes Perry Bowker of Port Carling, Ont. Therefore, reporting a snowfall of 250 millimetres is accurate, but 25 centimetres is easier to relate to. So is eight millimetres of rain, as opposed to 0.8 centimetres.