StormDebris:
The History of Meteorology
The Thermoscope to Thermometer: Measuring Heat and Cold
Date: c. 1590s–early 18th century
Location: Italy, Netherlands, England, and broader Europe
Type: Scientific instrument development
Author: Galileo Galilei; Santorio Santorio; Daniel Gabriel Fahrenheit; Anders Celsius
Why it matters: Enabled the quantitative measurement of temperature, transforming meteorology into an observational science
Timeline placement: The Instrumental Turn
Before temperature could be measured, it was understood only through direct sensation. Heat and cold were experienced but not expressed numerically. The transition from thermoscope to thermometer marks the point at which these qualitative impressions began to be treated as measurable quantities.
Emerging in the late sixteenth and early seventeenth centuries, early thermoscopes, often associated with Galileo Galilei, could indicate changes in heat but could not assign numerical values. Over time, these devices developed into thermometers, which allowed for standardized and repeatable measurement.
This shift was not only technical but methodological. It changed how atmospheric phenomena could be studied. Temperature became a variable that could be recorded, compared, and incorporated into broader systems of observation. The thermometer marks an early stage in what historians describe as the instrumental turn, when the study of weather began to rely on quantitative measurement.
Historical Context
By the late Renaissance, natural philosophy was undergoing a subtle but profound rearrangement. Observation remained central, but there was increasing emphasis on measurement to capture nature in stable, comparable terms. As historians such as Paolo Rossi have argued, this period saw the emergence of instruments not merely as aids to observation, but as tools that reshaped what could be known.
Heat and cold presented a particular challenge. Unlike length or weight, they resisted straightforward quantification. They were experienced directly but lacked a stable scale. Early thinkers, including Francis Bacon, emphasized the need to move beyond subjective sensation toward controlled observation, yet the means to do so were still emerging.
It is within this context that the thermoscope appeared. Often attributed to Galileo Galilei, though likely developed in collaboration or parallel with contemporaries, the device could register changes in heat through the expansion and contraction of air. According to accounts preserved by Santorio Santorio, thermoscopes were already in use in medical contexts, where physicians attempted to track patient conditions through changes in heat.
These early devices, however, lacked a crucial feature: a scale. They could show that something had become warmer or cooler, but not how much. The journey from thermoscope to thermometer would hinge on solving this problem of standardization.
An illustration showing an early thermoscope alongside alchemical glassware. Instruments such as these used the expansion and contraction of air to indicate changes in temperature before standardized numerical scales were developed.
The thermoscope operated on a simple principle: air expands when heated and contracts when cooled. In a typical design, a glass bulb connected to a tube was inverted into a liquid. As the air inside the bulb warmed, it expanded and pushed the liquid downward; as it cooled, the liquid rose.
The thermoscope could indicate whether temperature was increasing or decreasing, but it could not provide numerical measurements.
The transformation into the thermometer required two conceptual and technical advances:
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Sealing the system
Open-air thermoscopes were influenced by both temperature and atmospheric pressure. Sealing the liquid within the instrument reduced the effect of pressure changes and allowed temperature to become the primary variable being measured. This led to the development of liquid-in-glass thermometers, in which a liquid expanded and contracted within a sealed tube.
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Introducing a scale
The addition of fixed reference points allowed temperature to be measured consistently. Early thermometers used a variety of liquids, including water and alcohol, but these substances had limitations such as uneven expansion or low boiling points. Mercury became widely used because it expanded in a more uniform way, remained liquid over a broad range of temperatures, and was easy to see inside a glass tube.
As described by Daniel Gabriel Fahrenheit, reliable thermometry depended on reproducible calibration points. Fahrenheit developed a mercury thermometer with a standardized scale based on fixed reference temperatures, which improved consistency across different instruments. His scale was widely adopted in parts of Europe, particularly in England and the Netherlands.
Later, Anders Celsius proposed a centigrade scale based on the freezing and boiling points of water. The original version of this scale was defined with 0 representing the boiling point and 100 the freezing point; it was later reversed into the form commonly used today. The Celsius scale became standard in much of continental Europe.
Through these developments, thermometers became more consistent and comparable across different locations. Heat and cold were no longer treated only as qualitative states but as measurable quantities that could be recorded and analyzed systematically.
What It Proposed
Portrait of Anders Celsius, the Swedish astronomer who proposed a temperature scale based on the freezing and boiling points of water. His centigrade system later evolved into the modern Celsius scale used widely in scientific and meteorological observation.
Strengths and Insights
The development of the thermometer represents a turning point in the history of meteorology. Most importantly, it transformed temperature into a quantifiable variable. This allowed observations to be recorded, shared, and compared in ways that qualitative description could not support.
As Hasok Chang has emphasized in his study of temperature, early thermometry did not simply refine existing knowledge; it created a new kind of knowledge. Temperature became something that could be stabilized through instruments and conventions, rather than inferred from sensation.
The introduction of fixed points and standardized scales also enabled coordination. Observers in different locations could now compare readings, laying the groundwork for systematic weather records. This shift made it possible to detect patterns over time, contributing to the emergence of climatology.
Finally, the thermometer exemplifies the broader methodological shift of the period: from explanation grounded in qualitative categories to one increasingly structured by measurement and reproducibility. It is a small instrument with large consequences, turning the invisible fluctuations of heat into something legible.
Limitations and Errors
Early thermometers, despite their promise, faced significant challenges. Calibration was not initially standardized, and different makers used different reference points, making comparisons difficult. The choice of liquid, including air, water, alcohol, or mercury, also affected sensitivity and reliability.
Moreover, the relationship between temperature and physical processes was not yet fully understood. Thermometry developed before the formal articulation of concepts such as heat transfer, energy, and thermodynamic equilibrium. As a result, measurements could be precise without being fully explained.
Environmental factors also posed problems. Exposure to sunlight, wind, or enclosure could influence readings, and standardized measurement practices took time to develop. As historians have noted, the reliability of temperature data depended not only on instruments but on protocols such as where and how measurements were taken.
These limitations reflect a broader pattern: instruments do not simply reveal nature; they require interpretation, calibration, and agreement. The thermometer’s authority emerged gradually, built through practice as much as theory.
Historical Impact
The thermometer helped redraw the map of meteorology. Where earlier traditions relied on descriptive accounts of weather, the introduction of temperature measurement enabled systematic observation. Over time, networks of observers began recording temperature alongside other variables, such as pressure and precipitation.
This development played a central role in the emergence of modern meteorology. By the 18th and 19th centuries, temperature records contributed to the identification of seasonal patterns, regional climates, and long-term trends. The atmosphere, once described in qualitative terms, became a system that could be tracked numerically.
The thermometer also influenced other sciences. In physics, it became essential to the study of heat and energy; in medicine, it provided a diagnostic tool; in chemistry, it enabled controlled experimentation. Its reach extended far beyond weather.
The journey from thermoscope to thermometer illustrates a broader transformation. It is not simply the story of a device, but of a shift in how nature was approached. Measurement did not replace observation; it reconfigured it, giving rise to a new kind of precision.
If Meteorologica represents an approach based on qualitative explanation, the thermometer marks a shift toward understanding the natural world through quantitative measurement.
Related Pages
Timeline
This development marks the transition into instrument-based observation.
Themes
The thermometer contributes to the quantification of atmospheric variables.
Later Developments
The thermometer contributes to later theories in meteorology.
Sources & Notes
Primary Sources
Santorio Santorio. Commentaria in Artem Medicinalem Galeni. National Library of Medicine. https://catalog.nlm.nih.gov/discovery/fulldisplay?docid=alma992430903406676&context=L&vid=01NLM_INST:01NLM_INST&lang=en&adaptor=Local%20Search%20Engine&tab=LibraryCatalog&query=creator,exact,Santorio,%20Santorio,AND&facet=creator,exact,Santorio,%20Santorio&mode=advanced&offset=40
Fahrenheit, Daniel Gabriel. Experimenta circa gradum caloris liquorum nonnullorum ebullientium instituta (1724). Royal Society (Philosophical Transactions). https://royalsocietypublishing.org/rstl/article/33/381/1/110681/I-Experimenta-circa-gradum-caloris-liquorum?searchresult=1
Celsius, Anders. Observations of two persistent degrees on a thermometer (1742). Referenced in National Library of Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC7120475/
Secondary Sources
Chang, Hasok. Inventing Temperature: Measurement and Scientific Progress. Oxford University Press, 2004. Available via Internet Archive.
Rossi, Paolo. Philosophy, Technology, and the Arts in the Early Modern Era. Available via Internet Archive.
Middleton, W. E. Knowles. A History of the Thermometer and Its Use in Meteorology. Johns Hopkins Press, 1966. Available via Internet Archive.
Notes
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Attributions of the thermoscope to Galileo are traditional but debated; multiple early modern figures contributed to its development.
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Early thermometers varied widely in construction and calibration; standardization emerged gradually over the 17th and 18th centuries.
Revision Note
Last reviewed: April 2026