For his new orangery with its annexes, the solarium (sun house) and the vaporarium (steam house), Linnaeus required a functional thermometer in order to control the environment in the best possible way for his valuable plant material.

However, Linnaeus’ interest in the climate requirements of plants can be found already in 1737 and 1739 in his reports on his observations with a Fahrenheit thermometer from George Clifford's garden in Holland. This is long before he was appointed Professor of medicine and Head of the botanical garden at Uppsala University. In the quite extensive description of Clifford's garden by Linneus, Hortus Cliffortianus (1737), there is quite an interesting frontispiece inserted, rich in allusive details. This is a beautiful allegorical plate, including many interesting details, for example the two little boys (putti) at the bottom of the figure, pointing to a very special thermometer (Fig. 1).

The temperature scale was performed after the late professor of astronomy Anders Celsius' well defined natural fixpoints. In his famous work Observationer om twänne beständiga Grader på en Thermometer (1742), (Observations about two fixed degrees on a thermometer), he used the boiling- and freezing-points of water to obtain his temperature scale after most careful measurements. In Museum Gustavianum it is still possible to see one of the thermometers (de l'Isles' thermometer) which he used to establish his fix-points. Celsius' own thermometer instrument with its linear scale was of glass and designed for more experimental purposes starting with 0° at the boiling-point (D) and 100° at the freezing-point (C) of water (Fig. 3). This implied a practical method to measure, e.g., weather conditions, with no plus or minus. Linnaeus however, as a scientist, had the original Celsius scale turned, so that 0 (zero) was marked for the freezing-point and 100 for the boiling-point of water for his instrument (Fig. 2). By that Linnaeus got a thermometer better suited for biological conditions. Plants risk to die at 0° C. The celsius-scale, altered in this way, gives the reading below zero for minus-degrees and above zero for plus-degrees. But it also gives a much wider use, as one can even measure extreme temperatures as, e.g., the meltingpoint of iron (+1535°C). With Celsius' original thermometer-scale this would have been quite difficult.

Linnaeus was also eager to supply his disciples with the new instrument. When Pehr Kalm went to North America in 1747 and Anton Rolandson Martin to Spitzbergen in 1758 they were both supplied with this new thermometer.

Another one of Linneus' disciples, Anders Sparrman, supplemented his original observations in Fahrenheit-degrees with the new Celsius-degrees, so that he could give both readings in his original journal from South Africa.

In Linnaeus’ day a thermometer was indeed not in everyone's possession. The price was as much as 30 Swedish copper dalers which was about a month's salary for a blacksmith, or the price of a musket. In comparison we may note that a hunting rifle today costs about 10.000 SEK or 1000 EUR. No wonder Linnaeus was anxious that the University should approve the purchase. This has then given us evidence of his great interest in the thermometer also from a technical point of view by the letters and minutes that are still preserved today. It is also clear from them that Linnaeus was the very first to cause a thermometer to be functionally designed, measuring in degrees Celsius in the way we are now used to, already in 1744. The thermometer we use today is thus what you might call a "Linnaeus-thermometer.” In its growing triumphal progress throughout the world it has been called Celsius-novum or just the Swedish thermometer and in our days the Celsius thermometer.

Once again, it should be stressed that it is the combination of Celsius' ingenious method of determining the fixpoints and Linnaeus’ initiative as to the functional design of the thermometer which together form the basis of the first thermometer using modern Celsius degrees (C°).