224 OUTLINES OF NATURAL PHILOSOPHY. 



314. As heat is known to us principally by its 

 power of expanding bodies, we may take the ex- 

 pansion of some known substance as a measure of 

 the variations of heat. 



The substance selected for measuring heat is Mercu- 

 ry ; and the Thermometer is an instrument indica- 

 ting the variations of bulk to which we conceive the 

 variations of heat to be proportional. 



It has been found, that the variation of the bulk of 

 mercury, from being immersed in freezing, and in 

 boiling water, bears always the same proportion to 

 the whole bulk ; and, therefore, that variation di- 

 vided into a certain number of parts, may be taken 

 as a scale of the degrees of heat. 



In FAHRENHEIT'S thermometer, the interval above na- 

 med is divided into 180, and numbered at the 

 lower end 32, and at the upper end 212. In the 

 Centigrade or Centesimal Thermometer, the above 

 interval is divided into 100, and the degrees are 

 numbered from the freezing point. 



The word temperature is used to denote the heat indi- 

 cated by any degree of the thermometer, as exist- 

 ing in the air or in any other body. 



315. All bodies, whether solid or fluid, are ex- 

 panded by heat ; but not all in the same propor- 

 tion. 



2 Expansion 



