The instruments used in measuring temperature have been described in another chapter. From these instruments are obtained the current temperature of the air, the wet bulb temperature (used to compute the humidity), and the maximum and minimum temperatures of the day. Readings are made at fixed hours, known as “term hours.” At regular stations of the United States Weather Bureau the term hours for the observation of all the meteorological elements are 8 a. m. and 8 p. m., Eastern Time, and an observation of temperature, humidity, and clouds is made at noon. In most other countries tri-daily readings have been the rule, though in Europe four or more observations a day are now taken at many stations in order to supply the frequent weather bulletins required by aeronauts. Important stations are generally equipped with thermographs, which make a continuous record of temperature. Theoretically, the mean temperature of any day is the average of 24 hourly observations, from midnight to midnight. In practice, the mean is generally computed from the observations at the term hours, or from the maximum and minimum. Having obtained the mean daily temperature for each day of a month, the average of these values gives us the mean monthly temperature. The average of the mean temperatures for the twelve months of the year is the mean annual temperature. These data for each day and month, and for the year--sometimes also for other intervals, such as five-day periods, or “pentads,”--are computed year after year, and eventually the values for all the years of the record are averaged to form what are called “normals.” We thus obtain, for example, for a given station, the normal temperature for January 21, the normal temperature for the month of March, the normal annual temperature, etc. All this is a mere beginning toward the complete discussion of a body of temperature observations for the purposes of climatology. We have still to obtain from the readings of the maximum and minimum thermometers the normal maximum and minimum temperatures and range of temperature for each day, each month, and the year; also the “absolute” maxima, minima, and ranges (i. e., the extreme values that have occurred during the entire record) for corresponding intervals of time. These data furnish answers to such questions as: What was the lowest temperature ever recorded on January 21? What is the lowest on an _average_ January 21? What is the average range of temperature in March? What was the highest temperature ever recorded, on any day, at the station?