54 2 The Smithsonian Institution 



tion of the sounding body, should be as great as possible; and 

 the "quantity" of sound, depending on the magnitude of the 

 vibrating surface, should also be great. 



The audibility also depends on the state of the atmosphere, 

 the best condition being that of perfect stillness and uniformity 

 of density and temperature. The most efficient cause of the 

 loss of audibility is the direct effect produced by the wind. 

 While, as a general rule, the audibility of a sound is greater 

 on the side toward which the wind "blows," this is due to 

 downward refraction, rather than to the simple " carrying 

 effect " of the wind, which would hardly be sensible. Further- 

 more, there are instances of a greater audibility on the wind- 

 ward side, which is to be explained by reference to a domi- 

 nant upper wind, opposite in direction to that near the earth's 

 surface. 



There is not much utility in concave reflectors or other de- 

 vices for " directing " the sound along certain lines, for the 

 tendency of the wave is to spread with great rapidity, so that 

 within a distance of three or four miles of the source it fills 

 the whole space of air within the circuit of the horizon and 

 is heard nearly, if not quite, as well behind the trumpet as 

 before it. Neither fog, snow, rain, nor hail materially inter- 

 feres with the transmission of loud sounds, the siren having 

 been heard at a greater distance during the prevalence of a 

 dense and widely-extended fog than during any other condi- 

 tion of the atmosphere. Projecting portions of the land or 

 buildings may produce sound shadows, so that a sound easily 

 heard at a distance may be inaudible on nearer approach. 

 The existence of an " aerial echo " was established, the ex- 

 planation of which was not easy to see, although it is probably 

 due to reflection from the surface of the sea. 



Investigations of a similar character have been prosecuted 

 in a more or less irregular fashion by the Lighthouse Board 



