Everybody knows the siren because of alarm exercises. In such device air is pressed through the holes in a fast rotating disc. The air vibrations thus formed cause the characteristic, penetrating tone. In 1819 the apparatus was devised by Charles Cagniard de Latour (1777-1859), who called the device a ‘siren’. He was looking for a device which could show a connection between the pitch and the frequency (the number of air vibrations per second). The rotational speed of the disc can be read from a counter. The frequency can be calculated by means of a stopwatch. The produced tone sounds louder at high speeds because of the air driving. The apparatus was made by the French instrument maker Deleuil.