REPORT OF THE SECRETARY. 53 



an increase of velocity of the sound-wave in one direction and a dimi- 

 nution in the other by the motion of the wind except in an imperceptible 

 degree; for since sound moves at the rate of about seven hundred and 

 fifty miles an hour, a wind of seven miles and a half an hour could in- 

 crease or diminish the velocity of the sound-wave only one per cent., 

 while the effect observed is in some cases several hundred per cent. It 

 is, however, due to a change in its direction. Sound moving with the 

 wind is refracted or thrown down toward the earth; while moving 

 against the wind it is refracted upward and passes over the head of the 

 observer, so as to be heard at a distance at an elevation of several hun- 

 dred feet when inaudible at the surface of the earth. 



4. Although, as a general rule, the sound is heard farther when mov- 

 ing with the wind than when moving against it, yet in some instances 

 the sound is heard farthest against the wind, but this ijhenomenon is 

 shown to be due to a dominant upper wind, blowing at the time in an 

 opposite direction to that at the surface of the earth. These winds are 

 not imaginary productions invented to explain the phenomena, but 

 actual existences, established by observation, as in the case of the ex- 

 periments made at Sandy Hook in 1874, by means of balloons, and from 

 the actual motion of the air in the case of northeast storms, as observed 

 at stations on the coast of Maine. 



5. Although sound issuing from the mouth of a trumpet is at first con- 

 centrated in a given direction, yet it tends to spread so rapidly, that at 

 the distance of three or four miles it fills the whole space of air inclosed 

 within the circuit of the horizon, and is heard behind the trumpet nearly 

 as well as at an equal distance in front of its mouth. This fact pre- 

 cludes the use of concave reflectors as a means of increasing the inten-, 

 sity of sound in a given direction ; for although at first they do give an 

 increase of sound in the direction of the axis, it is only for a compara- 

 tively short distance. 



6. It has been established, contrary to what has formerly been thought 

 to be the case, that neither fog, snow, hail, nor rain materially interferes 

 with the transmission of loud sounds. The siren has been heard at a 

 greater distance during the prevalence of a dense and widely-extended 

 fog than during any other condition of the atmosphere. This may, how- 

 ever, be attributed to the uniform density and stillness of the air at the 

 time. 



7. In some cases sound-shadows are produced by projecting portions 

 of land or by buildings situated near the origin of the sound, but these 

 are closed in by the spread of the sound-waves, and thus exhibit the 

 phenomenon of sound being heard at a distance and afterwards lost on 

 a nearer approach to the station. 



8. It frequently happens that on a vessel leaving a station the sound 

 is suddenly lost at a point in its course, and after remaining inaudible 

 some time, is heard again at a greater distance, and is then gradually 

 lost as the distance is farther increased. This phenomenon is only ob- 

 served when the sound is moving against the wind, and is therefore 



