Now we saw nothing but sky and water and realized the omnipotence of God, into which we commended ourselves

From Empire of Blue Water by Stephan Talty.

The origin of the term dead reckoning.

There were no charts to guide Morgan, no way of measuring longitude. Navigation in the New World was an art that drew on ships' logs, lead lines (for measuring the ocean's depth), collective memory, and gossip. Dead reckoning was also a primary tool; sailing due east or west from a "deduced" position (or "d'ed" in the log, thus the term "dead reckoning") was a reliable method: Sail due east from the Canary Islands and you would arrive at Africa's west coast; sail west and you would find yourself in the Bahamas. But this kind of knowledge built up over decades; the West Indies had few such routes available to the captain. IN the Gulf of Honduras, ships that had become hopelessly lost in the foul weather were reduced to listening into the night for the splash of migrating tortoises, the only thing that could lead them to land. Ships' pilots prayed fervently to the Holy Virgin for guidance through a nest of reefs. Most pirates could attest to the truth of what a French soldier bound for the New World wrote in his journal, "Now we saw nothing but sky and water and realized the omnipotence of God, into which we commended ourselves."