Beelzebub's Tales to His Grandson

41 The Bokharian Dervish Hadji-Asvatz-Troov

41.890

“‘This siren was invented two centuries ago by a certain learned physician named Zehbek and it was so to say perfected in the middle of the last century, by a certain Cagniard-de-la-Tour.

41.890

“‘The construction of this childish bauble consists in this, that a current of condensed air is directed from a pipe on to a revolving disc drilled with little holes, each hole exactly coinciding in size with the opening of the main air pipe; and as this disc revolves, the passage for the current of air, entering these holes from the main pipe, is alternately opened and closed.

41.890-1

“‘And thus during the rapid revolution of this disc, successive shocks of air are obtained in the holes, and these produce an even-pitched tone of sound, and the number of revolutions recorded by the clock mechanism, multiplied by the number of the holes of the disc, should give the number of the vibrations of that sound made in the given interval of time.

41.891

“‘Unfortunately for the Europeans, neither the first inventor nor the perfecter of that siren knew that sound can be obtained not only from the action of genuine vibrations but also from the simple flow of air; and as this siren of theirs sounds only from the flow of air and not at all from natural vibrations, therefore the determination of the exact number of vibrations by the indications of that siren is out of the question….

“‘And, that sound can be produced from two causes, namely, from natural world-vibrations themselves and simply from the flow of air, is a very satisfying and interesting fact, and I shall now demonstrate it to you practically.’