Rare chart of the Western or Atlantic Ocean showing the variations in compass readings.
The double line from the Cape Verde islands to Bermuda shows where the reading of the compass stands true. Cartographic elements include lines of latitude and longitude, compass rose, and location of shipping routes. Decorative elements include cormorants or birds, icebergs, and ships.
As explained in the two columns of pasted text, these observations are based upon the work of Dr. Edmond Halley in 1700 with additions in 1744. The map was originally published by William Mount, John Mount and Thomas Page in "The English Pilot. The Fourth Book" circa 1750 and re-issued thereafter for the next 30 to 40 years, including an addition by Mount & Davidson.