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Sheila,
 
The longitude in that instance is measured only east of the prime meridian,
from 0 to 360.  As for the prime meridian used for that map, I would expect
it to be "Ile de Fer" or, variously, "Ferro," or "Hierro" which was
originally defined as 20 degrees west of Paris, and is about 17:40 west
of Greenwich.  To determine the Greenwich "western" equivalent the
following simple computation is used
 
      360 - 280 = 80    (degrees west of PM used)
      80 + 17:40 = 97:40W  (degs west of Greenwich, assuming Ferro)
 
Remember that in 1748 longitude values were approximate.
 
Chris Baruth
AGS Collection