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57 6 Sir gegrge siiuckburgh’s Obfermtiom 
ftriking off the laft cypher to the right hand for a frac- 
tion; add together the two numbers thus found in the 
table of proportionable parts, and their fum i'ubdudt 
from the tabular numbers juft found in table II.; the 
differences of the tabular numbers, fo diminifhed, will 
give the approximate height in Englifh feet. 
Precept the 3d, Add together the degrees of the two 
detached or air-thermometers, and divide their fum by 
2, the quotient will be an intermediate heat, and muft 
be taken for . the mean temperature of the vertical co- 
lumn of air intercepted between the two places of ob- 
servation: if this temperature fhould be 31°^ on the 
thermometer, then will the approximate height, before 
found, be the true height ; but if not, take its difference 
from 3 1 0 ^, and with this difference feek the correction 
in table IV . for the expanfion of air, with the number of 
degrees in the vertical column on the left hand, and the 
approximate height to the neareft thoufand feet in the 
horizontal line at the top; for the hundred feet ftrike off 
one cypher to the right hand ; for the tens ftrike off two ; 
for the units three: the fum of thefe feveral numbers 
added to the approximate height, if the temperature be 
greater than 31°^, fubtra&ed if lefs, will give the correCt 
height in Englifh feet. An example or two will make 
this quite plain. 
example 
