246 OUTLINES OF NATURAL PHILOSOPHY. 



when this is attempted in different states of the at- 

 mosphere, it is found that the values of m are the 

 same when the mean temperature of the columns 

 of air intercepted between the stations at which the 

 observations are made, is the same; but that the 

 values of m are different, when the mean tempe- 

 ratures are different. It has been found, that when 

 h is to be expressed in English fathoms, and the 

 common tabular logarithms are employed, and if the 

 mean temperature be that of freezing, m = 10000, 

 so that the difference of the tabular logarithms, 

 counting the first four places integers, gives the 

 height in fathoms. 



For other temperatures, the value of m involves the 

 expansion of air for one degree of heat, which has 

 been taken at .00243, or .00244, as determined by 

 the experiments of General ROY and Sir GEORGE 

 SHUCKBURGH. This, however, does not perfectly 

 agree with the expansion as stated above, 328., 

 where it appears to be only .00222. A correction 

 is also required for the temperature of the mercury 

 itself; for that being rarely the same in the two ba- 

 rometers, a reduction is necessary to bring them both 

 to one temperature. 



341. If, therefore, b be the height of the mercu- 

 ry in the barometer at the lowest station, @ at the 

 highest, t and t' the temperatures of the air at 

 those stations, f the fixed temperature at which no 

 correction is required for the temperature of the 

 air ; and if q and q' be the temperatures of the 

 quicksilver in the two barometers, and n the ex- 

 pansion 



