ABSOLUTE AND APPARENT EXPANSION. 781 



a coefficient of linear expansion, but only of their coeffi- 

 cient of cubical expansion. The coefficient of cubical 

 expansion increases for most liquids very rapidly with 

 the temperature ; only for mercury it is approximately 

 uniform within a considerable range of temperature ; 

 between and 100 it is =0-00018153, or ^LIT- This 

 is the coefficient of absolute expansion ; that is, that 

 which would be observed if the vessel in which the 

 the mercury is contained did not expand at the same 

 time. The coefficient of apparent expansion of mercury 

 contained in a glass vessel in other words, that which 

 is in this case actually observed is less, and amounts 

 only to i-^Vo ? f r the coefficient of absolute expansion 

 of a liquid is very nearly equal to the coefficient of 

 apparent expansion of the liquid together with the 

 coefficient of cubical expansion of the substance of 

 the vessel in which the liquid is contained. 



The change in the specific gravity of mercury 

 caused by changes of temperature must be taken into 

 account when the pressure of the atmosphere is to be 

 accurately determined by the barometer. In order to 

 make the indications of this instrument comparable in 

 different places and at different times they are ' reduced 

 to ' ; that is, the height of a column of mercury at 

 is calculated, which would exert the same pressure as 

 that which is indicated by the barometer at the tem- 

 perature at which the observation is actually made. 

 This ' correction of the barometric height ' may be made 

 according to the following rule : Multiply the height 

 of the barometer by 5509, and divide the product by 

 the sum of 5509 and the number of degrees indicated 

 by the thermometer '; the quotient is then the corrected 



