But numerous voyages which were soon undertaken made known the general contour of the American coast-line, and the discovery of the Great South Sea by Balboa revealed at length the true facts of the case, and the mistake into which both Toscanelli and Columbus had fallen, that in a voyage to the West the distance from Europe to Asia could not exceed the distance passed over in a voyage from Italy to the Gulf of Guinea--a voyage that Columbus had repeatedly made.
In his first voyage, at nightfall on September 13, 1492, being then two and a half degrees east of Corvo, one of the Azores, Columbus observed that the compass needles of the ships no longer pointed a little to the east of north, but were varying to the west.

The deviation became more and more marked as the expedition advanced.

He was not the first to detect the fact of variation, but he was incontestably the first to discover the line of no variation.

On the return-voyage the reverse was observed; the variation westward diminished until the meridian in question was reached, when the needles again pointed due north.

Thence, as the coast of Europe was approached, the variation was to the east.