548 EESEAECHES IN SOUND. 



two experiments, tbe sound was heard slightly. The vessel then altered 

 its course so as to steam, as it were, around the signal, keeping at the 

 same distance until the direction of the station fi?om the vessel was 

 nearly at right angles to the direction of the wind ; at this point no 

 sound was heard from the station, although it had been slightly heard 

 at points along the curved line traversed in reaching the point men- 

 tioned. The vessel then proceeded toward the station in a straight line, 

 but no sound was heard until it approached the latter within one-fourth 

 of a mile. The observers at the station however heard the sound from 

 the vessel through the whole distance. 



This experiment was made to ascertain the truth of the general impres- 

 sion that at this -place the sound was always heard better coming at right 

 angles or across the wind than in the direction in which it was blowing. 

 The experiment however was found in conformity with the general rule 

 previously established, that the sound was usually heard farthest with 

 the wind, less against the wind, and at an intermediate distance across 

 the wind. 



The primary object of these investigations is to determine the mechan- 

 ical causes to which the phenomena may be referred and from which 

 new conclusions may be deduced, to be further tested by experiment, 

 and such definite views obtained as may be of value in the employment 

 of fog-signals for the uses of the mariner. 



For this i)urpose a number of different hypotheses may be provision- 

 ally adoj)ted and each compared with the actual facts observed. 



The first hypothesis which has been suggested for the explanation of 

 the phenomena in question is that they are due to some configuration of 

 the land ; but on inspecting the Coast-Survey chart of this region it will 

 be seen that the nearest land consists of a series of broken surfaces not 

 rising above the ocean enough to reflect sound or in any way to produce 

 sound-shadows in the region through which the phenomena are observed. 

 This hypothesis therefore is inadmissible. 



Another hypothesis is that of what have been called invisible acoustic 

 clouds or portions of atmosphere existing over the water at the region 

 of silence, which might absorb or variously refract the sound. That 

 such a condition of a portion of the atmosphere really exists in some 

 cases is a fact which may be inferred from well-established principles of 

 acoustics, as well as from experimental data. They would occur espe- 

 cially in the case of dissolving clouds, which would be accompanied by 

 local diminutions of temperature, and also from portions of air which 

 have been abnormally heated by contact with warm earth. But, if the 

 phenomena in question were produced by a cloud of this kind, its pres- 

 ence ought to be indicated by transmitting through it the usual set of 

 meteorological instruments. This was done in the foregoing exi)eri- 

 ments, but no change was observed in the indications either of the 

 thermometer or barometer. Unfortunately we had not a hygrometer in 

 our possession, but this observation was less necessary, since from- 



