An hourglass filled with fine sand, poured through a tiny hole,at a constant rate, indicated a predetermined passage of time.
An hour glass is basically 2 bubbles of glass with a narrow middle; wood is used to close off the sand. The sand is measured and sealed and the hour glass is turned over and over. This type of clock was the first one to not be dependent on the weather. It's used for short periods of time such as speeches, sermons, watch duty, cooking, and at sea to calculate one's position.
To calculate the speed at sea, one would throw a piece of wood overboard tied with a knotted rope. When a knot ran through one's fingers in 1/2 a minute measured by the hour glass, it indicated that the vessel was going at the speed of 1 nautical mile an hour. The knots were very wide apart and one just counted the knots. Hence the phrase "knots an hour".
The center of the hour glass would get clogged. Course sand wore away at the center and made the opening wider. An hour glass had to be on a flat surface in order to work properly.