In reading this thread, my question was why the scale starts arbitarily with zero at some point, with observable phenomena occuring at 32 and 212 degrees. So I asked Wikipedia.
Quoth Wikipedia:
"There are a few competing versions of the story of how Fahrenheit came to devise his temperature scale. According to Fahrenheit himself in an article he wrote in 1724 [1] he determined three fixed points of temperature. The zero point is determined by placing the thermometer in a mixture of ice, water, and sodium chloride (or sea salt). This is a type of frigorific mixture. The mixture automatically stabilizes its temperature at 0 degrees F. He then put an alcohol or mercury thermometer into the mixture and let the liquid in the thermometer descend to its lowest point. The second point is the 32nd degree found by mixing ice and water without the salt. His third point, the 96th degree, was the level of the liquid in the thermometer when held in the mouth or under the armpit. Fahrenheit noted that, using this scale, mercury boils at around 600 degrees.
His measurements were not entirely accurate; by his original scale, the actual melting and boiling points would have been noticeably different from 32 °F and 212 °F. Some time after his death, it was decided to recalibrate the scale with 32 °F and 212 °F as the exact melting and boiling points of plain water. That change was made to easily convert from Celsius to Fahrenheit and vice versa, with a simple formula. This change also explains why the body temperature once taken as 96 °F by Fahrenheit is today taken by many as 98.6 °F (it is a direct conversion of 37 °C), although giving the value as 98 °F would be more accurate"