[Illustration: [From the _United States Coast Survey Report_, 1880, No. 84.]]
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.
[Illustration: [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.
[Sidenote: Columbus's misconception of the line of no variation.]
[Sidenote: Sebastian Cabot's observations of its help in determining longitude.]
As he still moved westerly the magnetic line was found to move farther and farther away from the pole as it had before the 13th approached it. To an observer of Columbus's quick perceptions, there was a ready guess to possess his mind. This inference was that this line of no variation was a meridian line, and that divergences from it east and west might have a regularity which would be found to furnish a method of ascertaining longitude far easier and surer than tables or water clocks. We know that four years later he tried to sail his ship on observations of this kind. The same idea seems to have occurred to Sebastian Cabot, when a little afterwards he approached and passed in a higher latitude, what he supposed to be the meridian of no variation. Humboldt is inclined to believe that the possibility of such a method of ascertaining longitude was that uncommunicable secret, which Sebastian Cabot many years later hinted at on his death-bed.
The claim was made near a century later by Livio Sanuto in his _Geographia_, published at Venice, in 1588, that Sebastian Cabot had been the first to observe this variation, and had explained it to Edward VI., and that he had on a chart placed the line of no variation at a point one hundred and ten miles west of the island of Flores in the Azores.