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FunctionThis is a set of wet and dry bulb thermometers, which is useful for determining the humidity of the air. The dry bulb thermometer is a standard thermometer with its mercury bulb exposed to the air. The wet bulb thermometer has its mercury bulb covered in cloth and during its use, this cloth would be kept moist by contact with a water reservoir. The wet-bulb thermometer gives lower readings than the dry one due to the cooling effect of the evaporation of water. This cooling is proportional to the dryness of the air (both temperatures being the same when the air is 100% saturated with water vapor). One can find out the humidity of the air using a table and the readings for dry and wet bulb temperature.
ProvenanceFrom the Blue Hill Meteorological Observatory.