Intelligence and Miscellaneous Articles. 485 



same column of mercury, at the same temperature, estimated in 

 English inches, at the standard temperature of 62° F. 



From Guyot's Tables for the conversion of millimetres into English 

 inches, and vice versa, we express millimetres at the standard tem- 

 perature of 0° C. in terms of English inches at the standard tempe- 

 rature of 62° F. 



The consequence is that the reading of the French barometer, when 

 reduced and converted into English inches by Guyot's Tables, ought 

 always to coincide exactly with the reading of the English barometer. 



From the following further quotation from Mr. Packe's paper, it 

 is easy to see how he has arrived at a different conclusion : — 



" For exact observation, therefore, it is useless to have a barometer 

 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°-67 C). 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 mer- 

 cury — the expansion of the scale : 



mm. 



Expansion of mercury for 16°' 67 C. = 2' 2 12 

 Expansion of brass scale for ,, = — '231 



V981 



mm. 



736-59 

 — 1-981 



reduced 734-61 = 28-9224 inches." 



Mr. Packe evidently supposes that in a barometer with a double 

 scale, when the attached thermometer is at 62° F. = 16°' 67 C, and 

 the English reading is 29 inches, the metrical reading will be 736*59 

 millims. 



Now, if the metrical scale is properly graduated, this will not be the 

 case. 736'59 millims. is what the reading would be if the metrical 

 scale were at its standard temperature of 0° C. But by hypothesis 

 it is at 62° F. ; it has therefore expanded through the space due to 

 an increase of temperature of 30° F., that is, through -23 millim. 



If, then, the English reading be 29 inches, the corresponding me- 

 trical reading will be 736 # 36 millims. 



mm. 



736-36 

 Reduction for 16°- 67 C. 1-98 



Reduced reading . . 734'38 = 28-913 inches; 



