Marine sandglass

A marine sandglass is a special tool that sailors use to help them keep track of time while at sea. It looks like a big glass bottle with two bulbs (round parts) connected by a narrow tube in the middle. One bulb is on top, and the other is below it. The inside of the glass is filled with sand, tiny little bits of rock.

Here's how it works: When the sailor wants to measure a specific amount of time, say 30 minutes, they turn the glass over so that the sand falls from the top bulb to the bottom bulb. As the sand falls, it passes through the narrow tube, which takes 30 minutes to empty all the sand from the top bulb to the bottom bulb. After 30 minutes, all the sand has moved from the top bulb to the bottom bulb, and the sailor knows that 30 minutes have passed.

Marine sandglasses were important tools for sailors before modern clocks and watches were invented. They helped sailors keep a reliable schedule while at sea, which was especially important for navigation and communication. Even today, some sailors still use marine sandglasses as a backup to their more modern timekeeping devices.