The putti (the Italian word describing chubby male children, usually nude and often with wings, depicted in works of art in the early modern era) shown here were evidently helping to make such a hygrometer: one wielded scissors to cut some strands of the hair of the other for use in the instrument. In the standard version of the hair-tension hygrometer, a long strand of hair, under tension with a thread and weight, expands and contracts in length with changes in humidity. After describing what called his "new comparable hygrometer," de Saussure addressed theories of hygrometry and barometry and their application to meteorology.
For more on de Saussure and other 18th-century natural philosophers who made of the weather a kind of experimental physics. see, for example, Theodore Feldman, "Late Enlightenment meteorology," in The quantifying spirit in the 18th century (Berkeley: University of California Press, 1990), 143-177. And J. L. Heilbron explores the role of putti in the depiction of experiments in "Domesticating science in the eighteenth century," in Science and the visual image in the Enlightenment, ed. William R. Shea, European studies in science history and the arts, 4 (Canton, Mass.: Science History Publications, 2000), 1-24.
