Remaj'Tcs on the Nature of Sound in Water. 9S 



as I have said, at a distance of less than 200 metres, the sound 

 of the bell struck under water is easily heard in the air ; but, 

 at a greater distance, its intensity diminishes very rapidly, until 

 at length, at the distance of 400 or 500 metres, it is impossible 

 to distinguish the slightest sound, even very near the surface of 

 the water. However, on immersing the head a few centimetres, 

 or by putting down a tube filled with air, as I did, the sound 

 of each blow is heard strongly and distinctly, and in this man- 

 ner it is heard at a distance from ten to twenty times greater. 

 It is evident, that, at a distance of 500 metres, the vibrations ar- 

 rived at the surface under a sensible angle, which was further in- 

 creased by the curvature of the earth. The vibrations which 

 take place in water, do not therefore communicate with the air, 

 when their direction meets the surface at a small angle, — a pheno- 

 menon analogous to that presented by the surface of separation 

 of two mediums of different densities. 



The agitation produced by the waves does not alter the du- 

 ration of sound nor its velocity, when a tube is used for hearing. 

 The last of the three experiments mentioned above was made in 

 stormy weather. The wind, which at first was weak, increased 

 to such a degree, that several anchors were necessary to hold the 

 vessel. Notwithstanding the noise of the waves, I could still 

 distinguish pretty well the sound of each stroke, and the dura- 

 tion of its transmission was not altered. 



The last observation which I have to make, relates to the 

 influence of screens on the intensity of sound. Having chosen 

 two stations, at no great distance from each other, and so situat- 

 ed that the straight line which joined them grazed the extremity 

 of a thick wall which rose above the level of the water, I had 

 the bell struck regularly, and with strokes of equal intensity. 

 Hearing them with the tube alternately on either side of the 

 line which grazed the extremity of this wall, it appeared to me 

 that there was a very marked difference of intensity, according 

 as this extremity was or was not interposed between the bell 

 and the tube. The transmission of sound in water, therefore, 

 differs in this respect from what takes place in air, and ap- 

 proaches to the mode of the propagation of light. This influence 

 of a screen insensibly diminishing the intensity of sound, deserves 

 to be remarked, and affords a new point of approximation be- 



