Aneroid barometer

Small vessels most often use an aneroid barometer.
Although it is not as accurate as a mercury barometer, it is more robust, compact and affordable.
The aneroid barometer consists of a corrugated container nearly exhausted of air, with a spring inside, along with a lever, a pointer and a graduated dial.
When air pressure increases, the top of the corrugated container bows in, and when air pressure decreases, the top of the corrugated container bows out.
The lever transmits these changes to the pointer on a dial.
Air pressure can be measured in inches of mercury or in millibars, also known as hectopascals.