[Gk. thermos, hot; and metron, measure.] An instrument for measuring changes of temperature by the contraction or expansion of a liquid or a gas. The three scales at present in use are - (1) the Fahrenheit, in common use in Great Britain and the United States ; (2) the Centigrade, used on the Continent and in scientific works generally ; (3) Reaumur's scale, used in Russia. In Fahrenheit's scale the freezing-point is marked 32°F. and the boiling-point 212°F. the intervening space containing 1800. In the Centigrade scale the space between the two points is divided into 100 equal parts-the freezing-point being marked 0° C., and the boiling point 100° C. In Reaumur's scale the freezing-point is marked 0° R., and the boiling-point 8o° R., the space between the freezing and the boiling-points being divided into 80 equal parts. Since 1800 on the Fahrenheit scale correspond to 100° on the Centigrade, the length of one degree Fahrenheit is 100-180th or 59th of one degree Centigrade, and any reading on the Fahrenheit scale is converted into the corresponding Centigrade reading by the following rule: Subtract 32 and multiply the remainder by 5-9th. When very low temperatures are required an alcohol thermometer is used, because mercury freezes at about -380 F. Air is of great use in determining temperatures above those at which mercury can be employed (mercury boils at 66o° F.). Other types of thermometers are maximum, minimum, and solar radiation or self-registering thermometers. (See Fahrenheit, Reaumur.)