The Central England Temperature (CET) record, a meteorological dataset originally published by English climatologist Gordon Manley in 1953, and subsequently extended and updated in 1974 following many decades of painstaking work, documents the monthly mean surface air temperatures, for the Midlands region of England in degrees Celsius from the year 1659 to the present. This record represents the longest series of monthly temperature observations in existence. It is monthly from 1659, and a daily version has been produced from 1772.
"The monthly means from November 1722 onwards are given to a precision of 0.1°C. The earliest years of the series, from 1659 to October 1722 inclusive, for the most part only have monthly means given to the nearest degree or half a degree, though there is a small 'window' of 0.1 degree precision from 1699 to 1706 inclusive. This reflects the number, accuracy, reliability and geographical spread of the temperature records that were available for the years in question" (Wikipedia article on Central England temperature, accessed 03-09-2013).
Manley, G., "The mean temperature of central England, 1698–1952," Quarterly Journal of the Royal Meteorological Society, vol. 79 (1953) 242-261.