NAVIGATION. 131 



68. An old instrument called the siren, in- 

 vented by Cagniard de la Tour, for the purpose 

 of illustrating the science of sound, has been 

 recently taken up by the United States Light- 

 house Board with great success, as a sub- 

 stitute for the fog horns previously used at 

 lighthouses in foggy weather. The siren, in 

 its original form, is an instrument in which a 

 hole or holes in a flat side or top of an air 

 vessel, are alternately obstructed and opened by 

 the revolution of a disc of metal, perforated 

 with a number of equidistant holes in a circle 

 round its axis. Air blown constantly into the 

 vessel escapes alternately in abundance, and 

 but slightly, as the holes are alternately opened 

 and obstructed by the revolution of the disc ; and 

 thus a musical note is produced, with a pitch 

 precisely determined by the number of openings 

 and closings per second of time. Instead of a 

 little instrument, suitable for a lecture-room table, 

 both turned and having its blast supplied by a 

 small acoustic wind-chest and bellows, the 

 Americans have made a powerful instrument with 



K 2 



