@John-c5143186-eeea-4ca7-8853-866785408ff8
It has to do with humidity.
For centuries humidity was measured by using (horse's) hair!
https://www.scienceworld.ca/resource/hair-hygrometer/

https://collection.sciencemuseumgroup.org.uk/objects/co54855/de-saussure-hair-hygrometer-1815-1841-hygrometer-case-container

https://ashraeiowa.org/committees/history/attic-antiques/horse-hair-humidistat/