As early as 1625 an Italian physician called Sanctorius noticed that there appeared to be a close connection between illness and high body temperature.
No one took much notice for about 200 years, but by the middle of the 19th century there had developed a universal recognition that core body temperature was an important indicator of the state of any person's health. A practical difficulty, however, prevented the usefulness of this idea from being exploited.
To monitor the progress of their feverish patients, doctors were obliged to use large and cumbersome thermometers, instruments which could take up to 20 minutes to register the correct temperature. There was also the added problem for even the most nimble and careful of physicians that the registered temperature might well have changed before the reading could be noted.
Both problems were solved by the clinical thermometer designed in 1866 by Dr Thomas Allbutt. Since the variation of the body's temperature is limited, the clinical thermometer does not need to have a large range; it has a scale which runs from 35 to 42 degrees, which allows a certain compactness of design.
But Allbutt's instrument also incorporated a clever device to capture the temperature of a patient long enough for it to be observed accurately and recorded. It had a small bend or constriction in the tube that prevented the mercury from returning to the bulb, allowing the doctor ample time to take a reading.
"O imitatores, servum pecus!" rages Horace in his Ars Poetica: "O imitators, what a slavish herd you are!" And he might well have been speaking of the sequestration of the clinical thermometer by meteorologists. They use a variation of it to measure the highest temperature each day.
To keep a constant eye on the thermometer, making sure to spot the exact instant when it hits its highest point, would be much too tedious for meteorologists. Instead they use a maxi- mum thermometer, very similar to the instrument that doctors use, but capable of measuring a much larger range of temperatures.
As the air temperature rises during the morning, the thermal expansion of the mercury forces it through the narrow opening leading to the tube until the highest value of the day is reached. As the temperature begins to fall again, however, there is no pressure on the fluid to force it back past the constriction; the mercury column breaks, and the instrument continues to register the value of the highest temperature.
The instrument is reset every morning so that it can register the maximum temperature of the day, which usually occurs some time in the early afternoon.