THERMOMETERS. 99 



50. Thermometers. As a general rule, bodies expand 

 under the action of heat, and contract under the action of 

 cold, and the only method of measuring temperatures is by 

 observing the extent of the expansion or contraction of 

 some known substance. Any body which indicates changes 

 of temperature may be called a thermometer. 



As the expansions of different substances are not exactly 

 proportional to one another, it is necessary to select some 

 one substance or combination of substances to furnish a 

 standard, and the standard usually adopted for all ordinary 

 temperatures is the apparent expansion of mercury in a 

 graduated glass vessel ; for very high temperatures, a metal 

 of some kind is the more useful, and for very low tempera- 

 tures, at which mercury freezes, alcohol must be employed. 



The mercurial thermometer is formed of a thin glass tube 

 of uniform bore, terminating in a bulb, and having its upper 

 end hermetically sealed. The bulb contains mercury, which 

 also extends partly up the tube, and the space between the 

 mercury and the top of the tube is a vacuum. Since the 

 glass, as well as the mercury, expands with an increase of 

 temperature, the apparent expansion is the difference be- 

 tween the actual expansion and the expansion of the glass. 

 The construction of an accurate mercurial thermometer is 

 an operation of great delicacy. 



In Fahrenheit's Thermometer, which is chiefly used in 

 England and in this country, the freezing point is marked 

 32, and the boiling point 212. The space, therefore, be- 

 tween these two points is 180. 



In the Centigrade Tliermometer the freezing point is 

 marked 0, and the boiling point 100, the space between 

 being divided into 100. 



In Reaumur's Tliermometer the freezing point is also 

 marked 0, but the boiling point is marked 80 *. 



* The temperature indicated by the boiling point is the same in all. 



