Siren is an instrument which produces a sound by converting a steady current of air or some other gas into a series of discontinuous puffs. This may be done by an arrang ement such as that shown in the illustration. A B (Fig. 1) is a cylinder whose top consists of a disc C D perforated by a number of holes. Another disc E F (Fig. 2) will exactly fit over C D, and by means of a pin in the centre P, which can rest in slight hole at H, the top disc can rotate quite freely over the lower one; The number of holes in E F and C D is the same, but their directions are different. If c d and a b (Fig. 3) be holes in the lower and upper disc, a stream of air sent up c d will strike the side a of the upper orifice and so cause the movable disc to rotate. This effect is proportional to the number of perforations in the two discs, since they are all superposed at the same moment. By merely sending air up the tube into the cylinder the upper disc rotates, and, as it moves, the holes in upper and lower discs are constantly changing from positions of coincidence to disagreement. Hence the air can only issue from E F in puffs, and these puffs produce a certain note. The siren can be used to determine the number of vibrations in a note produced by any means; it is then provided with an apparatus which registers the number of revolutions in any time, and with an adjustment for increasing or decreasing the number of orifices. When the siren is producing a sound exactly in agreement with the other, the number of vibrations per second in both is the number of revolutions of the disc per second multiplied by the number of holes in use. Sirens are frequently used as fog-horns on ships and in lighthouses.