Page:Handbook of Meteorology.djvu/193

 Weather Bureau Thermometers.—Several kinds of thermometers are necessary for the requirements of a weather service; and these are practically the same in all parts of the world where a weather service is maintained. In weather stations the daily maximum and the daily minimum are required; a continuous graphic record is desirable, but the instrument for this purpose is supplied to stations of first-class equipment only.

A standard thermometer of the ordinary type, that is, one. which shows existing temperature at any time, is desirable. A minimum thermometer can be used for this purpose, but a standard instrument is preferable. The scale, divided to single degrees, should be engraved on the tube and on the metal



The maximum thermometer is so called from the fact that the mercury in the tube is not drawn back into the bulb when the temperature lowers. The expansion of the mercury in the bulb forces the flow into the bore, as with ordinary thermometers. A slight constriction of the bore at the top of the bulb prevents a backflow, thereby leaving the mercury in the bore at the maximum temperature since its last previous setting. Usually the maximum temperature of the day occurs between 2:30 and 4:00  and the thermometer should be set late in the