i ago 3 
RoguOealaeab. 
Set the index to the complement of the latitude 40° 30, and 
mark the diftance failed om the perpendicular, and move it 
until it meets the index, then the point of interfection will 
fhow the difference of longitude 210 miles, or 3° 30 on the 
index. 
Ir it fhould happen that the meridional parts of the given 
latitudes (36° and 44° N. lat.) are not contained on the fame 
line of the cylinder, but begin upon one: and. terminate on 
another, then this difficulty is removed by fliding the cylinder’ 
until the meridional degree of the latitude in 36° oo’ is brought 
near the extremity of the bafe, fixing the femicircle perpen- 
dicularly over it, and moving the edge of the perpendicular to 
the termination of the graduations on the cylinder, then turn 
the cylinder until the continuation of the line required ‘appears, 
and place the beginning of it to the edge of the perpendicular 
where the former part of the line terminated, and move the per- 
pendicular until it cuts the degree required, and the fpace inter-- 
cepted between it and the femicircle will be the meridional: 
difference of lJatitude fought. 
Fhe 
