As far as we know, Celsius never met his rival Fahrenheit. The former was Swedish, and died in 1744; Gabriel Daniel Fahrenheit, on the other hand, was German, and had been born in Danzig a generation earlier - 313 years ago today in fact, on May 14th, 1686. He was the originator of the scale of temperature that bears his name.
There is some doubt as to how exactly Fahrenheit arrived at his invention. Thermetrorum scala, he wrote in 1724, quae meteorologicis observationibus solum modo inserviunt, infra a Zero incipit et 96to gradu finitur: "The scales for thermometers that are to be used solely for meteorological observations begin at zero at the bottom, and extend up to 96 degrees".
Concerning the higher value, he went on to explain that "the alcohol expands to this height when the thermometer is placed in the mouth, or in the arm-pit, of a healthy living man so as to acquire fully the heat of the body."
According to one account, he had first decided to graduate the thermometer using minus 90 as the coldest temperature obtainable, zero as moderate, and plus 90
as the greatest heat possible. But in the end he came up with something completely different.
Here is what he had to say of it himself: "The division of the scale depends upon three fixed points which are obtained in the following manner: the first point, at the lower end of the scale, was found by a mixture of ice, water and sal ammoniac, or also seasalt; when a thermometer is put into such a mixture the liquid falls until it reaches a point which we call `zero'.
"The second point is obtained if water and ice are mixed without the salts named; when a thermometer is put into this mixture the liquid stands at 32. The third point is at 96; the alcohol expands to this height when the thermometer is placed in the mouth, or in the arm-pit, of a healthy man and held there until it acquires the temperature of the body."
Interestingly, Fahrenheit did not use the boiling-point of water as one of his fixed points; he merely extended his scale, and the division marked "212" just happened to be the appropriate one. But the number was considered highly significant at the time: it meant that the range of temperature over which water was in the liquid state amounted to 180, and there are 180 in a semi-circle.
This apparent coincidence, everyone was sure, must hide some very important fundamental natural law, but no one, alas, ever found out precisely what it was!