380 Elements of Mineralogy. 



The barometrical heights, of both barome- 

 ters being thus corrected, the logarithm of the 

 number of inches at which the mercury in the 

 upper ftands, is to be fubftracted from the lo- 

 garithm of that of the lower ; the difference 

 between the 4 firft decimal figures on the left 

 gives nearly the number of fathoms, as that of 

 the two laft decimals does of the parts of a 

 fathom between the upper and lower barome- 

 ters, which being multiplied into 6, gives the 

 number of feet and parts of a foot. 



I faid nearly r , for this height is not exact, 

 except when the mean temperature of the 

 atmofphere is 3??,24 of Fahrenheit. 



The mean temperature of the atmofphere 

 is the arithmetical mean between the degrees, 

 marked by two detached thermometers, one 

 above, and the other below, both being held 

 for about an hour in the ftiade. 



When this medium is 3^,24, the above 

 method is juft, and requires no further cor- 

 rection, but if the medium exceeds, or falls 

 fliort of 3i?,24 that difference muft be found, 

 fuppofe itz:^. 



The approximate height muft now be cor- 

 rected by multiplying it in feet, into the 

 fraction 0,00243, let the product/*, then 



