The lid is perforated with a series of holes all the same size, all slanted in the same direction (about 45°), all at the same distance one from the other and all on two equal circumferences.
Above the lid and almost in contact with it there is a brass disk on a vertical, revolving steel shaft passing through its centre and in grazing contact with the lid.
The disk has a series of identical holes which are slanted in the opposite direction with respect to those of the lid. The two series of holes can be superimposed perfectly.
When the siren is placed on a bellows box with the two series of holes perfectly aligned, the jet of air from all the holes on the lid strikes the walls of the corresponding hole on the disk and, owing to the opposite slant, it begins to turn in the direction of the air jet.
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