508 WRITINGS OF JOSEPH HENRY. [1877 



tion to that at the surface of the earth. Such winds are 

 not imaginar}^ productions invented to explain the pheno- 

 mena, but actual existences — established by observation, as 

 in the case of the experiments made at Sandy Hook, in 1874, 

 by means of balloons, and from the actual motion of the 

 the air in the case of north-east storms, as observed at sta- 

 tions on the coast of Maine. 



5. Although sound issuing from the mouth of a trumpet 

 is at first concentrated 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 within the circuit of the hori- 

 zon, and is heard behind the trumpet nearly as well as at 

 an equal distance in front of its mouth. This fact precludes 

 the use of concave reflectors as a means of increasing the 

 intensity of sound in a given direction ; for although they 

 do give an increase of sound in the direction of the axis, it 

 is for only a comparatively short distance. 



6. It has been established (contrary to what was formerly 

 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 dur- 

 ing the prevalence of a dense and widely-extended fog than 

 during any other condition of the atmosphere. This may 

 be attributed to the uniform density and stillness of the air 

 at the time. 



7. In some cases sound-shadows are produced by project- 

 ing portions of land or by buildings situated near the origin 

 of the sound; but these shadows are limited in extent, and 

 are closed in at some distance by the spread of the sound- 

 waves, thus exhibiting the phenomenon of sound being 

 heard at a distance when lost on a nearer approach to the 

 station. 



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

 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 in- 

 creased. This phenomenon is observed only when the 

 sound is moving against the wind, and is therefore attrib- 



