English and French Barometers. 9 



duced to the freezing-point, the scale of the French barometer 

 is also reduced to the freezing-point, but the scale of the English 

 one is only reduced to the temperature of 62° F. 



The consequence is that the French barometer, when reduced, 

 will always read higher than the English barometer. 

 Let A be the height of the barometer observed ; 



B the linear expansion of brass for 1° F. =-0000104344, 

 as given by Laplace and Lavoisier, or '000018782 for 

 1°C. 



The French barometer, when reduced, will, on account of the 

 difference of standard temperatures, read higher than the English 

 barometer by an amount =30 AB; e. g., 



Let the height of the barometer be = the boiling-point 212°, 

 i, e. 29905 inches. 



B=-0000104344= log 5-018467 



A = 29-905 = log 1-475743 



30 = log 1-477121 



3-971331 = -009361 inch excess of 

 the French reading. 



By changing A it will be evident that we get the excess for any 

 height of the barometer, but for the average height at the sea- 

 level it may be taken as -009 inch. 



For exact observation, therefore, it is useless to have a baro- 

 meter marked with a double scale, — the French and English : 

 they cannot be made to coincide ; e. g. 



Let the barometer read 29 inches = 736*59 millims. (temp. 

 62°F. = 16°-67C.). In the English scale at 62° (the temperature 

 of the standard) no correction is made for the brass scale. The 

 only correction is for the expansion of the mercury — *087, 



in. 



29 

 - -087 



reduced 28-913 = 734-38 millims. 



But in the French scale, the temperature of the standard being 

 32° F., the correction to be made is for the expansion of the 

 mercury — the expansion of the scale : 



mm. 



Expansion of mercury for 16°'67 C. = 2-212 

 Expansion of brass scale for „ = — -231 



1-981 



mm. 



736-59 

 - 1-981 



reduced 734'61 = 28'9224 inches. 



