visit the thermometer

The Thermometer

At the start of the seventeenth century there was no way to quantify heat.
In Aristotelian matter theory, heat and cold were fundamental qualities.
Like dry and wet, heat and cold were qualities combined with "prima materia"
to make up the elements, earth, water, air, and fire. Thus earth was dry and
cold, fire dry and hot, etc. Although one might speak of "degrees of heat or
cold," there was no formal distinction between what we would call the
extensive concept of heat and the intensive concept of temperature. Also
these degrees were not measured, except perhaps in a very rough way as when
a physician put his hand on a patient's forehead and diagnosed "fever heat."

Measuring heat became a puzzle in the circle of practical and learned men in
Venice to which Galileo belonged. The first solution was a thermoscope.
Building on Pneumatics by Hero of Alexandria (1st century BCE), first
published in the West in 1575, several authors had begun playing with the
idea of the expansion of air as its heat increased, and vice versa. The
first versions, usually called thermoscopes, were little more than toys.
Benedetto Castelli wrote in 1638 about a device he had seen in Galileo's
hands around 1603:

     He took a small glass flask, about as large as a small hen's
     egg, with a neck about two spans long [perhaps 16 inches] and
     as fine as a wheat straw, and warmed the flask well in his
     hands, then turned its mouth upside down into the a vessel
     placed underneath, in which there was a little water. When he
     took away the heat of his hands from the flask, the water at
     once began to rise in the neck, and mounted to more than a
     span above the level of the water in the vessel. The same Sig.
     Galileo had then made use of this effect in order to construct
     an instrument for examining the degrees of heat and cold. [1]

Over the next several years this thermoscope was developed by Santorio
Santorio and Galileo's friend Gianfrancesco Sagredo (both in Venice),
Galileo, and others to include a numerical scale. It had thus become a
full-fledged air thermometer. The first series of quantitative
meteorological observations date from this period. In other parts of Europe
the inventor Cornelis Drebbel and Robert Fludd developed similar
instruments. The questions about who was the first, and whether one derived
his knowledge from another, are sterile ones which shed little light on the
historical context in which this and other instruments (e.g., the telescope
and barometer) developed. The near simultaneous (and surely independent)
invention of the air thermometer illustrates the seventeenth-century trend
toward quantification of natural phenomena--an essential dimension of the
"mathematization of nature."

The liquid in glass thermometer was developed in the 1630s, but a universal
standard of temperature remained elusive. Each scientist had his own scale
divisions, often based on different reference points. It is impossible for
us accurately to convert their measurements to our temperature scale, and at
the time it was impossible to compare temperatures in different places. In
the early eighteenth century, universal temperature scales based on several
fiduciary points (e.g. a mixture of ice and brine, a mixture of ice and
water, body temperature, the boiling point of water) were developed by
Daniel Gabriel Fahrenheit (1686-1736), Anders Celsius (1701-1744), and
René-Antoine Ferchault de Réaumur (1683-1757). Of these, the first two are
still in use, and the system of Celsius (extended to become an absolute
scale in the nineteenth century) has become the standard scientific
temperature scale.

Thermoscope from the Multimedia Catalog of the Institute and Museum of the
History of Science in Florence, Italy
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Notes
[1]Le opere di Galileo Galilei, vol XVII, p. 377. I have used the
translation in W. E. Knowles Middleton, A History of the Thermometer and its
Use in Meteorology, p. 8.
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Sources
The standard work on the history of the thermometer is W. E. Knowles
Middleton, The History of the Thermometer and its Use in Meteorology
(Baltimore: Johns Hopkins University Press, 1966). Information on
Fahrenheit, Celsius, and Réaumur can be found in the Dictionary of
Scientific Biography.
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galileo@rice.edu by Albert Van Helden