SAM OGDEN / SCIENCE PHOTO LIBRARY SAM OGDEN / SCIENCE PHOTO LIBRARY
Thermohydrographs. Two designs of thermohydro- graphs, instruments for making and recording measurements of the temperature and relative humidity of the air. It contains a thermometer and a hygrometer. The thermometer leaves a trace on the upper portion of the paper chart; this upper portion is calibrated in units of degrees Fahrenheit (F). The hygrometer leaves a trace on the lower half; relative humidity is measured as the ratio of water vapour in the air to that in air which is saturated with water vapour at the same temperature.
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