SECULAR CHANGE OF THE AGONIC LINE IN THE NORTH ATLANTIC BETWEEN 1500 AND 1900.
[From the United States Coast Survey Report, 1880, No. 84.]
Variation of the needle.
A knowledge of the variation of the needle came more slowly to be known to the mariners of the Mediterranean. It had been observed by Peregrini as early as 1269, but that knowledge of it which rendered it greatly serviceable in voyages does not seem to be plainly indicated in any of the charts of these transition centuries, till we find it laid down on the maps of Andrea Bianco in 1436.
LAPIS POLARES MAGNES.
[From Hirth's Bilderbuch, vol. iii.]
It was no new thing then when Columbus, as he sailed westward, marked the variation, proceeding from the northeast more and more westerly; but it was a revelation when he came to a position where the magnetic north and the north star stood in conjunction, as they did on this 13th of September, 1492.
Columbus's misconception of the line of no variation.
Sebastian Cabot's observations of its help in determining longitude.