The log is an interesting invention. It began with 16th-century ships – surrounded with water, no land in sight, no sense of direction, and no sense of speed. What did you have to compare your speed with anyway?
The sailors were ingenious. They used a log – a real piece of wood – attached to a string. They’d throw it in the water and use it to measure speed.
Then they’d track this speed over time, in a logbook.
That’s the origin story of log books, now known as logs.
On Wikipedia, I also learned about the origin of the term knot as a speed metric.