A Brass Weather-glass: the Art of Glass and the Advent of Instrumental Meteorology

On 8 January, 1661/2 (old style) Sir Christopher Wren showed his colleagues in the newly formed Royal Society of London his design for a “Weather Clock.” This clock would be be a self-registering device recording multiple weather-related variables: wind, rain, dew, and the movement of a weather-glass (an air thermometer, which confounded temperature and pressure, as discovered at that time, documented by Edmund Halley in 1701). Wren’s recording weatherglass (a “Weather-Wheel”) was a clever contraption using a rotating drum which moved according to the movement of water in a circular tube. Assuming that the quality of glass blowing available to him in London was poor, Wren went to extraordinary means to have made by musical instrument makers a brass tube, a tube which required a special coating to preserve the brass from the mercury the weight of which turned the device as the contained air expanded and contracted.
The reconstruction of the mechanism according to the best guesses of seventeenth century scientists and twentieth century historians is that of a circular tube similar to the air-thermometers which were beginning to emerge in Italy, the Low Countries, and then in England. This device, more commonly known as the weather-glass (or vitrum calendare to Francis Bacon), generally consisted of an open-ended glass vessel containing a fluid (usually water) and a trapped air-pocket, displaying changes in heat and cold (later to be called “temperature”) by changes in the level of the fluid as the air expanded and contracted. (Had the earlier users of the weather-glass known about air pressure, they would have known they were seeing changes in that variable as well.)
The air thermometer is not an instrument in the modern sense. However, its popularity as novelty, a toy, a diagnostic medical device, a demonstration, and as a prognostic auger, along with the techniques of manufacture, and the material conditions which made it possible in turn helped engender instruments we would recognize as properly scientific, particularly the sealed thermometer and the various forms of the barometer, as well as other instrumental curiosities. The history of the air thermometer is shrouded in mystery, emerging at the end of the 16th century, perhaps developed independently in the Italian states and the Low Countries, the era of Galileo and Santorio (both among those credited for its invention), possibly based on a parlor trick, or a bit of popular prognostication. It also can be found in Cornelius Drebbel’s perpetual motion. Wren’s version was just a peculiar take on the air thermometer (perhaps inspired by Drebbel’s gimmick), was technically diffic
ult to accomplish in 1660s London—it was ahead of local abilities, and local knowledge.
The rise and diffusion of this instrument follows the diffusion glass making technology. Modern glass technique was considered a state secret in the Republic of Venice—leaving Venice with craft knowledge was a capital offense. By necessity, escaped journeymen would flee to places like the Netherlands or Bohemia, joining the glass industries in Northern havens. The late medieval period was an era of trade secrets, but this began to break down the rise of the printed book. In 1612, Florentine Antonio Neri published L'Arte vetraria, exposing many trade secrets, and starting a publishing tradition of over two centuries of translations and expansions of The Art of Glass. And with the diffusion of industrial technique, we see the diffusion of the instrument.
At the time Wren was working on his brass instruments, Royal Society colleague Christopher Merret published the first English language expansion, Art of Glass (1662), right at the time when that body was initiating instrumental weather observation in England, trailing the better equipped Florentine weather network by nearly a decade.
By the eighteenth century, manufacturing techniques had extended to France and northern Europe, and along with that, increasing levels of sophistication in data acquisition, data which, although not up to modern standards, still remains use for the reconstructions of historical climatologists.
Besides the intellectual traditions of Western Europe, the conditions for the development of meteorology in Europe as opposed somewhere else (be it China, per the Needham thesis, or any other developed or traditional culture), is based on growth of Western technologies: glass manufacture, moveable type, horology, cartography, and double-entry bookkeeping. The technology didn’t necessarily determine the rise of meteorology, but the technical conditions necessary for the “quantitative” thing that was modern meteorology seem to have been a perfect storm.