The longitude (abbreviation: Long., Î», or lambda) of a point on the Earth's surface is the angle east or west from a reference meridian to another meridian that passes through that point. All meridians are halves of great ellipses (often improperly called great circles), which converge at the north and south poles.
Use the dual thermometers to calculate relative humidity. Fisher Scientific Mason's Hygrometer has one dry thermometer and one wet thermometer which provide temperature readings for determining the relative humidity.
Other hygrometers work on the basis of a length of natural fibre, such as hair or a fine strand of gut, changing with variations in humidity. In a dew-point hygrometer, a polished metal mirror gradually cools until a fine mist of water (dew) forms on it. This gives a measure of the dew point, from which the air's relative humidity can be calculated.