In file a4_188.html, the Straight Dope classic from Dec 15, 1989, Cecil writes: “Romer had decided that the boiling point of water should be 60 degrees … [Fahrenheit] multiplied Romer’s degrees by 4 … Then … he multiplied all the numbers by 16/15”.
Now, 60 * 4 * (16/15) = 256.
So, why is the boiling point of water on the Fahrenheit scale 212 and not 256?
In his column, “On the Fahrenheit scale, why is 32 freezing and 212 boiling? What do 0 and 100 mean?”, Cecil says
Could it have been that Romer had miscalculated?
It sounds to me like Romer was just guessing at what the boiling point of water was. He knew that it was somewhere in (what we would now call) the range of 200 - 300 degrees Fahrenheit, but he couldn’t be any more accurate than that given his lousy intruments, especially at high temperatures. The value that he picked turns out to be what we presently measure as being 256 degrees Fahrenheit. He then picked what we now call 0 degrees Fahrenheit as to be the 0 point, just because he thought that was as low as he would ever have to measure. He then measured and found that freezing was at about 7 1/2 degrees on his scale and that body temperature (in so far as he could measure it) was at about 22 1/2 degrees.
Fahrenheit then got hold of his system and decided to change it by multiplying everything by 4, making the freezing point 30, the body temperature 90, and the boiling point 240 (and the 0 point stayed 0, of course). This not only made everything significant a whole number, but they were all divisible by 15. Fahrenheit didn’t quite like those numbers though, so he multiplied everything by 16/15, making the significant numbers now 0, 32, 96, and 256. This made everything significant divisible by 16.
When people finally got around to measuring the boiling point accurately, they found that it actually was about 212 Fahrenheit. Romer’s guess had been way off. It would have been at 49.6875 on his original scale and at 198.75 after Fahrenheit multiplied everything by 4. At that point, 32 degrees and 212 degrees were declared to be the freezing point and the boiling point by definition.
When Celsius created the centigrade system, he made freezing be 100 by definition and boiling be 0 by definition. People didn’t like the temperature scale going that way, so it was reversed to make freezing be 0 and boiling be 100.
When people finally got around to accurately measuring body temperature, they found that 96 Fahrenheit wasn’t very accurate. 37 degrees centigrade is accurate to within a half-degree centigrade. When this is translated to Fahrenheit, it equals 98.6. Actually, 98.4 is a better average estimate, but there’s a fair amount of random variation in the average body temperature.