]68 BULLETIN OP THE 



continued to increase in loudness until we reached the distance 

 of four miles ; at this point it was heard with such clearness that 

 the position of the station could be readily located in the densest 

 fog, but on proceeding still farther in the same direction it gradu- 

 ally diminished and was finally again lost. 



As a second experiment we retraced the same line back to the 

 station, and observed the same phenomena in a reversed order. 

 The sound was heard the loudest at a point about four miles 

 from the station, and after that diminished and became inaudible 

 through a space of about two miles, and then suddenly burst 

 forth nearly in full intensity at a distance of a quarter of a mile, 

 and continued loud until the station was reached. 



Now, for the investigation of this phenomenon, we may assume 

 provisionally that it is due to a peculiar condition of the atmo- 

 sphere, either as to heat, pressure, or moisture, or a combination 

 of all of them, which existed at the time in that part of the track 

 of the steamer which may be denominated "the region of silence." 

 But if this were true, such a condition of the atmosphere ought 

 to be indicated by ordinary meteorological instruments. To test 

 this, the temperature of the air was noted through the whole 

 space by an ordinary thermometer, and also its pressure by means 

 of an aneroid barometer, but no variation was observed in these 

 instruments in passing through the air along the path of the 

 vessel. 



To complete this series of observations, however, the indi- 

 cations of a delicate hygroscope should have been noted. Unfor- 

 tunately we were not provided with an instrument of this kind; 

 the fact however that the phenomenon was frequently observed 

 during a fog, or while the air is uniformly saturated with moist- 

 ure, indicates that the phenomenon is not due to a difference of 

 moisture in the region of silence. Indeed, it is sufficient to re- 

 member that a wind was blowing at the rate of ten miles an 

 hour, to be convinced that an isolated portion of air could not 

 remain in a fixed position, even for an instant. 



Another hypothesis might be assumed, — that the apparent 

 silence was caused by the transverse reflection, in some way, of 

 sound from the shore, but there was nothing in the configuration 

 of the land which favored such an hypothesis. The only ex- 

 planation which presented itself was that of the upward refraction 

 of sound, an hypothesis which has been found fertile in new re- 



