Amos Emerson Dolbear, an American physicist and naturalist, calculated the relationship between air temperature and the rate at which crickets chirp. Amos's calculation became known as Dolbear's Law:
°F = 50 + ( ( n - 40 ) / 4 ) OR °C = 10 + ( ( n - 40 ) / 7 )

Mr. Dolbear was a pioneer in nineteenth century telecommunications who also enjoyed a teaching career as a professor of Natural Science at the University of Kentucky in Lexington and at Bethany College in West Virginia during the 1860's and 1870's. Decades later, Amos published his study, The Cricket as a Thermometer in the periodical, The American Naturalist, Vol. 31, No. 371 in November of 1897. In his study, Amos wrote,

"At night, when great numbers (of crickets) are chirping, the regularity is astonishing for one may hear all the crickets in a field chirping synchronously....The rate of chirps seems to be entirely determined by the temperature and this to such a degree that one may easily compute the temperature when the number of chirps is known. Thus at 60 °F, the rate is 80 per minute. At 70 °F the rate is 120 a minute, a change of four chirps a minute for every degree."

Source: http://www.jstor.org/stable/2453256