The universal clinical procedure of recording a patient’s temperature depends upon the accuracy of thermometers. This in turn depends upon the accuracy of two fixed datum points (the freezing and boiling points of water) and subsequently on the fine calibration of the etched scale between them. Anders Celsius (1701–44) defined the boiling point of water as the upper fixed point of the thermometric scale, originally designated as 0°C but inverted by Carl Linnaeus (1707–78) to read 100°C. In 1724 Gabriel Fahrenheit (1686–1736) had observed that the upper fixed point, that of boiling water, varied with changes in atmospheric pressure. An English scientist, Sir George Shuckburgh (after 1794 known as Sir George Shuckburgh Evelyn), addressed this problem and over the period 1774–79 he defined the relationship of the temperature of boiling water to barometric pressure. This latter variable changed both with the ambient meteorological conditions of the moment and the height above sea level at which the calibrations were made. Clinical thermometry depends on an accuracy of 0.1°C in both the baseline and the tracking of a patient’s temperature but Shuckburgh’s experiments showed that the upper fixed point of reference, that of boiling water, could change by up to 10°C. He demonstrated that these variables must be measured and controlled in the manufacture and calibration of thermometers. Sir George Shuckburgh Evelyn published his results in the Philosophical Transactions of The Royal Society (1777–79) and made possible the accuracy of thermometry on which patient care depends.