Fahrenheit was a bookkeeping apprentice in Amsterdam when he became fascinated with thermometers being sold from Florence. The scales on these crude devices were arbitrary; the makers tended to mark the low point as the coldest day and the high point as the hottest day in Florence for the year a thermometer was built. Fahrenheit realized the folly of using arbitrary hot and cold points for the calibration and worked to develop thermometers using alcohol and mercury, with zero set at the coldest temperature he could achieve with salt in ice water, 32 at the freezing point of water, and 96 for the temperature of the human body. How did he get the 96? He used a simple scale with four major divisions between 0 and 32 and eight divisions of the same size between 32 and 96 -- too bad that the top temperature was not quite accurate, but it would have to do to avoid complexity in scale rulings. Since his original thermometers used alcohol as a working fluid, he could not put a point at the boiling point of water, since his alcohol would evaporate!  Fahrenheit made his mark in 1714 when he demonstrated two thermometers that gave the same readings!

Fahrenheit later did use the boiling point of water with his more advanced mercury thermometers. On the Fahrenheit scale, the freezing point of water is 32 degrees and the boiling point is 212 degrees.

To the right is an 18th century thermometer similar to those built by Fahrenheit. This one was constructed by a Frenchman, Reaumur, who developed his own temperature scale based on the freezing and boiling points of water.

from the Deutsches Museum, http://www.deutsches-museum.de/akt/presse/p0403_3.htm