152 INTENSITY OF SOUND. [SECT. xvi. 



Many instances may be brought in proof of the strength 

 and clearness with which sound passes over the surface of 

 water or ice. Lieutenant Forster was able to carry on* a 

 conversation across Port Bo wen Harbour, when frozen, a dis- 

 tance of a mile and a half. 



The intensity of sound depends upon the extent of the 

 excursions of the fluid molecules, on the energy of the trans- 

 ient condensations and dilatations, and on the greater or 

 less number of particles which experience these effects. 

 We estimate that intensity by the impetus of these fluid 

 molecules on our organs, which is consequently as the square 

 of the velocity, and not by their inertia, which is as the 

 simple velocity. Were the latter the case, there would be 

 no sound, because the inertia of the receding waves of air 

 would destroy the equal and opposite inertia of those ad- 

 vancing ; whence it may be concluded that the intensity of 

 sound diminishes inversely as the square of the distance 

 from its origin. In a tube, however, the force of sound does 

 not decay as in open air, unless perhaps by friction against 

 the sides. M. Biot found,, from a number of highly interest- 

 ing experiments made on the pipes of the aqueducts in 

 Paris, that a continual conversation could be carried on, in 

 the lowest possible whisper, through a cylindrical tube 

 about 3120 feet long, the tune of transmission through that 

 space being 2-79 seconds. In most cases sound diverges in 

 all directions so as to occupy at any one time a spherical 

 surface ; but Dr. Young has shown that there are excep- 

 tions, as for example when a flat surface vibrates only in 

 one direction. The sound is then most intense when the 

 ear is at right angles to the surface, whereas it is scarcely 

 audible in a direction precisely perpendicular to its edge. 

 In this case it is impossible that the whole of the surround- 

 ing air can be affected in the same manner, since the 

 particles behind the sounding surface must be moving to- 

 wards itj whenever the particles before it are retreating. 

 Hence in one half of the surrounding sphere of air its ino- 



