Time And Tide

This sign hangs outside Craiglea Clocks, where we went to collect our old GPO clock from being serviced. There are three faces, one for time, one for pressure and this, which appears to indicate tide movement. I don't understand how this works so if anyone can tell me, please do.

Mr Rafter not only mends clocks; he also makes musical instruments and demonstrated guitar, ukelele, mandolin and psaltery (an English relation of the zither) that he has made.