Used with an Anemometer to measure wind speed and direction. One needle shows the wind speed on the graph paper while the other indicates the wind direction. Measures atmospheric pressure. Air presses on a bulb of mercury, forcing it up a tube. Higher pressure increases the mercury level.
High pressure indicates good weather.
National Research Council of Canada, Instrument Division. Consists of one wet thermometer and one dry one. The evaporation of the liquid on the wet one lowers its temperature. The difference of the temperature between the thermometers indicates the relative humidity. Measures sunshine duration.