the Meafurement of Heights with the Barometer . 683 
obfervation on Mount ./Etna, one of the greateft acceffible 
heights in Europe, the difference of temperature at the top 
1 
and bottom might amount to 30% and this would occafion 
a difference of about five feet, which, I apprehend, may 
be reckoned inconfiderable in a height of 11,000 feet. 
In fa<ft, in an obfervation on this mountain by Mr. des- 
saussure it amounted to only 3^ feet. I may add, that 
your equation makes the computed height lefs than 
mine. 
I proceed to the expanfion of air. Your equation is 
various according to the circumftances, the difference 
therefore of our refults will, according to the circum- 
ftances, be various. The following table will give the 
quantity of this difference, viz. it fhews how much your 
refult is + or - mine upon one thoufand feet, according 
to different preffures of the atmofphere and different 
temperatures. The firft column to the left hand con- 
tains the mean heat of the column of air between the two 
barometers ; the figures in the horizontal line at top are 
the mean height of the two barometers, or mean preffure 
of the atmofphere ; the common point of meeting in the 
different columns gives the difference of our refult in 
feet, according to the refpe&ive circumftances. 
4P 2 
Mean 
