@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/