Bulb Hygrometer

“This hygrometer consists of two thermometers a and b similar to each other in all respects except that one of them has a piece of muslin tied closely over the surface of its bulb c, and kept constantly wet by a few threads of cotton which connect it with the water in the vessel d. the water then which rises from the vessel by capillary attraction spreads over the muslin, and evaporates from its surface with more or less rapidity according to the dryness or moistness of the air; and the greater the dryness of the air the greater is the difference between the observed readings of the dry and the wet thermometers.” — Encyclopedia Britannica, 1893

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