One of the most curious meteorology objects I’ve discovered recently is the weather glass. It was first described in 1558 by the Italian scholar Giambattista della Porta.
Della Porta’s apparatus was essentially the same as the air thermoscope, which I wrote about a recently. The alternative design shown below was in use from the 1600s. As the air in the vessel expands and contracts water moves up and down the spout, indicating changing atmospheric conditions.
Before air pressure was understood, the instrument was sometimes called a perpetuum mobile – perpetual motion – because the water level fluctuated with no known cause.
The English physician and mystic Robert Fludd (1574-1637) interpreted the weather glass as a ‘key to two worlds’. For him, it was a microcosmic symbol of the universe and a model for the human body. Others claimed that it could predict the weather days, weeks or even months in advance.
By the 1660s leading experimental philosophers, who had recently begun to distinguish between temperature and air pressure and to use the thermometer and barometer respectively to measure them, tended to dismiss the weather glass since it responded to both variables.
However, it remained attractive for domestic use due to its simplicity, and portability: one maker claimed in 1917 that his was ‘More Sensitive than the Most Perfect Barometer’. Weather glasses can be bought on ebay and are still popular with amateur weather forecasters today.
And whilst Fludd’s occult philosophy fell out of favour, some of his ideas persisted. Throughout the 1700s and 1800s many people continued to regard the weather glass and the barometer as reflections of the human body and psyche, since instruments and humans were both influenced by atmospheric conditions.