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Franklin noted a principle of refrigeration by observing that on a very hot day, he stayed cooler in a wet shirt in a breeze than he did in a dry one. To understand this phenomenon more clearly Franklin conducted experiments. On one warm day in Cambridge England in 1758, Franklin and fellow scientist John Hadley experimented by continually wetting the ball of a mercury thermometer with ether and using bellows to evaporate the ether. With each subsequent evaporation, the thermometer read a lower temperature, eventually reaching 7 �F (-14 �C). Another thermometer showed the room temperature to be constant at 65 �F (18 �C). In his letter �Cooling by Evaporation� Franklin noted that �one may see the possibility of freezing a man to death on a warm summer�s day.�