Imagine living sometime around the early 1600s. You're standing on the deck of a wooden ship. The wind's biting, the sky's endless. And your captain's yelling, "Drop the log!" Back then, before sonar ...

Mariners used to toss a wooden log overboard, and while its line spooled out, they would count each of the knots that passed through their hands. After 28 seconds, they recorded their boat’s speed in ...