BOOK II. 



ACOUSTICS-APPLICATIONS OF THE PHENOMENA 

 AND LAWS OF SOUND. 



CHAPTER I. 



SOUND-SIGNALS. 



I. ACOUSTIC SIGNALS IN NAVIGATION BELL-BUOYS SPEAKING- 

 TUBES THE INVISIBLE WOMAN. 



THE idea of using sound the human voice, bells or other similar 

 instruments to communicate at a distance is of very ancient 

 origin. The range of sound is doubtless infinitely less than that of 

 light, and light signals furnished a means of distant signalling long 

 before electricity brought this valuable and useful art to perfection. 

 But light is not visible or is only faintly seen during foggy 

 weather, or in the midst of storms : then sound is a useful 

 auxiliary which is employed at the entrance of ports or in the 

 vicinity of rocks. " In foggy weather," says M. Eenard, " ports are 

 signalled by bells rung at certain intervals. Some light-houses 

 are furnished with this apparatus. In the United States [and at 

 some places on our own coasts] where fogs are frequent and very 

 thick, notwithstanding the expense a wide range of sound necessi- 

 tates, at several points there are placed bells, weighing as much as 

 or more than 50'0 kilogrammes, and at others, whistles, fog-horns, 

 or syrens, worked by steam or compressed air." In narrow channels, 

 near banks or rocks, buoys furnished with bells to warn mariners 

 of danger, are often employed. Church bells, in country and in 



