To ascertain the longitude was a more difficult matter.

They were obliged to rely mainly on their dead reckoning; that is, to make a calculation of the course and distance run daily, from the points steered by the compass and the rate as indicated by the log-line and half-glass.

A reckoning on such a basis, where unknown currents prevail, where a vessel is steered wildly, or where the rate of sailing may be inaccurately recorded, is liable to many errors; therefore it was customary with all prudent masters, in those days, especially if they distrusted their own skill or judgment in keeping a reckoning to KEEP WELL TO THE EASTWARD.

This was a general rule, and looked upon as the key to West India navigation.

Sometimes a vessel bound to the Windward Islands, after reaching the latitude of her destined port, found it necessary to "run down," steering due west, a week or ten days before making the land.
An incident occurred in those waters, a few weeks after we passed over them, which will illustrate this mode of navigation, and the consequences that sometimes attend it.