Daniell hygrometer, used at Melbourne Observatory.

Consists of 2 thermometers, one immersed in a volatile liquid which by evaporation, lowers temperature to dewpoint. The other thermometer is fixed to the stand & records temperature of uncooled air. Originally invented by English chemist and meteorologist John Frederic Daniell in 1820.

A hygrometer measures the amount of humidity in the atmosphere.

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