A traditional unit of temperature still used customarily in the United States. The unit was defined by the German physicist Daniel G. Fahrenheit (1686-1736), who also invented the mercury thermometer. Fahrenheit set 0^o at the coldest temperature he could conveniently achieve using an ice and salt mixture, and it is said he intended to set 96^o as the temperature of the human body. (If so, he was off a little there; normal oral temperature for humans is between 98 ^oF and 99 ^oF.) On this scale, the freezing point of water (at normal sea level atmospheric pressure) turned out to be about 32 ^oF and the boiling point about 212 ^oF. Eventually the scale was precisely defined by these two temperatures. 1F equals 5/9 ^oC, but in converting between scales we have to be careful to adjust the zero points as well. To convert a temperature in ^oF to the Celsius scale, we must first subtract 32^o and then multiply by 5/9. In the other direction, to convert a temperature in ^oC to the Fahrenheit scale, we must first multiply by 9/5 and then add 32^o