Is a Cruise Ship's Horn/Whistle system mechanical or electrical or both?

We all know that a Cruise Ship has a horn and sometimes more than one.They are operated using compressed air. But, how does the system to activate the horn work, from the button on the Bridge to the horn/whistle on the funnel or radar mast.

Is it mechanical, (are there moving parts involved), is it electrical (does an electrical signal initiate a set of mechanisms to more)?