It was a sound device which was activated by the opening of the door which it protected, constituting the first sound alarm worldwide. It consisted of a trumpet adapted to a concave hemispherical container that was suspended from an articulated bar. With the opening of the door, a rope allowed the inclination of the bar and consequently the descent of the trumpet. The hemispherical container was immersed into a container with water and the enclosed air in this forced the trumpet to sound.
