eee 
\ 
To find the difference of longitude, fet the index to the com~ 
plement of the middle latitude on the femicircle, and move the 
perpendicular until the termination of the departure on it cuts 
the index, then the point of interfection will mark the difference 
of longitude 210 miles on the index. 
GAS E.. IV. 
Boru latitudes and departure from the meridian given, to find 
the courfe, diftance and difference of longitude. 
Lat. 56° 40’ S. Long. 28° 55’ E. failed from. 
Lat. 61° 20'S. arrived at; departure 172 miles. 
By Mercator’s failing. 
N. B. In failing through fouthern laticudes, the end of the 
cylinder where the numbers begin muft be turned towards the 
north, pointed out by the femicircle; and in failing through 
northern latitudes, it muft be reverfed. 
Rule.—Move the perpendicular and femicircle to the proper 
latitudes given, and lay the departure on the perpendicular, then 
move the index until it meets the point in which the depar- 
ture terminates, the diftance, will be marked on the index 329, 
and the courfe S. 31° 35’ E. or S.S. E,3 E. nearly, on the femi- 
circle, 
To 
