404, Mr. Greatorex’s observations, &c. 
4thly. Take the mean of the two detached thermometers ; 
and for every degree which this differs from 31 0 , take so 
many times the —g. part of the approximate height ; and add 
them if the mean temperature be above 31 0 ; but subtract them 
if it be below 31 0 ; and the sum or difference will be the true 
altitude in fathoms. 
