Marine Sandglass for 30 seconds

An hourglass was an instrument used in navigation to estimate time which, when used with a reel and line (known as a "chip log"), allowed a boat's speed to be measured in knots. It comprised a crystal or glass bulb, held in a wooden frame. The bulb narrowed at the center and was filled with sand or a similar material that flowed into the lower half as a result of gravity.

The hourglass was one of the instruments used most commonly on boats to measure time as it used solids rather than liquids, which could cause condensation problems with the changing temperatures. However, humidity did affect sand as well so, over time, hourglasses were improved by making them out of brass or iron, and by making them airtight. In navigation, these instruments were used to regulate life on board and to calculate speed.