A siren is a device used to produce a loud, piercing sound of very high pitch, which acts as a warning or a signal. It is powered by electric motors, steam or hand cranks. Sirens are used for various purposes. For example, in factories as a warning signal, indicating danger, accident, or at the start and closing time of the factory; for cautioning people in case of enemy air attack; for seeking traffic clearance by ambulances, police vehicles, etc. The siren was invented in the late 18th century by a Scottish natural philosopher, John Robinson. It was named by a French engineer, Charles Cognaird de La Tour, who devised an acoustical instrument in 1819, which is no longer in use today. The Cognaird siren consists of a cylindrical hollow box ‘C’, known as the wind chest. It has a fixed top lid ‘B’ that consists of a number of equidistant slanting holes arranged in circles. The chest is fixed at its lower end with a tube ‘T’, through which air is blown in by means of bellows. Another disc ‘D’ of the same size, with equal number of equidistant holes slanting in the opposite direction, covers the first disc. This disc is rotated by a mechanical arrangement. The air under pressure is forced into the wind chest of the siren due to which the disc begins to rotate. The air passes through the holes in the lower disc, and comes out through the holes in the upper disc. Every time the holes in the upper disc are above the holes in the lower disc, a puff of air escapes. This creates a sound. The pitch of the sound depends upon the speed at which the disc rotates. Today, improved versions of this siren are available. One such type of siren uses two small cylinders one inside the other, which have holes in them. An electric motor rotates the outer cylinder. Another device forces air or steam through the cylinders. The holes in the cylinders are actually small slots, which are cut at a slant encircling the cylinders. The air enters the slots in the first cylinder, and then passes through the slots in the second cylinder. Every time the slots are exactly opposite to each other, the movement of air through the slots creates the sound. Faster the rotation of the outer cylinder, louder is the sound.