Chip logs were used to measure a ship's speed through the water. A plate or other object tied to the end of the rope (the "chip") was thrown into the water, and the rope spooled out as the ship moved along. After a certain amount of time (typically measured by sand glass), the rope was hauled out of the water and the number of knots that came out was counted. This gave the ship's speed in "knots", now known as nautical miles.
Chip logs were an important navigational tool. In the days before reliable timekeeping, these were the best method for determining longitude. By combining the speed in knots and the amount of time the ship had sailed, the navigator could determine east/west distance with varying degrees of accuracy.
Mid-1500s to late 1800s.
- The Chip Log from The Mariner's Museum.
- Wikipedia's page on Chip Logs.
- A Chip Logs at Michael Hammond's history of science page.