Wet and dry bulb hygrometers, like the one above made by Fastre Aine in 1851, were more reliable than condensation hygrometers for measuring air humidity. First developed in 1755 by William Cullen (1710-1790) and Joseph Black (1728-1799), wet and dry bulb hygrometers measured humidity based on the evaporation of water.
Cullen and Black noticed that the level of a wet thermometer would drop slightly as it dried. With further studies, Cullen and Black observed how evaporation produced a coldness that could be measured. If one thermometer was kept in a wet muslin sleeve, indicating the temperature at which water evaporated, and a second thermometer measured normal air temperature, the difference provided a measurement of humidity. The rate of evaporation and the amount of cooling was determined by the amount of water vapour already in the air.