1877] WRITINGS OF JOSEPH HENRY. 521 



As a second experiment we re-traced the same line back 

 to the station, and observed the same effects in a reversed 

 order. The sound was heard the loudest at a point about 

 four miles from the station, and after that it diminished and 

 became inaudible through a space of about two miles, and 

 then suddenly burst forth nearly in full intensity at a dis- 

 tance of a quarter of a mile, and continued loud until the 

 station was reached. 



For the investigation of this phenomenon, we may as- 

 sume provisionally that it is due to a peculiar condition 

 of the atmosphere, 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 de- 

 nominated "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 tem- 

 perature 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 seiies of observations however the indi- 

 cations of a delicate hygroscope should have been noted. 

 Unfortunately we were not provided with an instrument of 

 this kind ; the fact however that the phenomenon was fre- 

 quently observed during a fog, or while the air is uniformly 

 saturated with moisture, indicates that the phenomenon is 

 not due to a difference of moisture in the region of silence. 

 Indeed, it is sufficient to remember 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 appar- 

 ent 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 explanation which presented itself was that of 

 the upward refraction of sound, an hypothesis which has been 



