Page:Works of the Late Doctor Benjamin Franklin (1793).djvu/142

132 An expedition againſt it was planned, and the command given to General Wolfe. His ſucceſs is well known. At the treaty in 1762, France ceded Canada to Great Britain, and by her ceſſion of Louiſiana, at the ſame time, relinquiſhed all her poſſeſſions on the continent of America.

Although Dr. Franklin was now principally occupied with political purſuits, he found time for philoſophical ſtudies. He extended his electrical reſearches, and made a variety of experiments, particularly on the tourmalin. The ſingular properties which this ſtone poſſeſſes of being electrified on one ſide poſitively and on the other negatively, by heat alone, without friction, had been but lately obſerved.

Some experiments on the cold produced by evaporation, made by Dr. Cullen, had been communicated to Dr. Franklin, by Profeſſor Simpſon of Glaſgow. Theſe he repeated, and found, that, by the evaporation of ether in the exhauſted receiver of an air-pump, ſo great a degree of cold was produced in a ſummer's day, that water was converted into ice. This diſcovery he applied to the ſolution of a number of phenomena, particularly a ſingular fact, which philoſophers had endeavoured in vain to account for, viz. that the temperature of the human body, when in health, never exceeds 96 degrees of Fahrenheit's thermometer, although the atmoſphere which ſurrounds it may be heated to a much greater degree. This he attributed to the increased perſpiration and conſequent evaporation, produced by the heat.

In a letter to Mr. Small of London, dated in May 1760, Dr. Franklin makes a number of obſervations, tending to ſhew that, in North America, north-eaſt ſtorms begin in the ſouth-weft parts. It appears, from actual obſervation, that a north-call ſtorm, which extended a