1875] WRITINGS OF JOSEPH HENRY. 447 



PART IV.— INVESTIGATIONS IN 1875. 

 (Report of the United States Light-House Board for 1875, pp. 104-126.) 



Preliminary Remarks. — In the appendix to the Light-House 

 Report of 1874 I gave an account of a series of investigations 

 relative to fog-signals which had been made at different 

 times under the direction of the chairman of the committee 

 on experiments. 



These investigations were not confined to the instruments 

 for producing sound, but included a series of observations on 

 sound itself in its application to the uses of the mariner. In 

 the course of these investigations the following conclusions 

 were early arrived at : 



1st. That the rays of a beam of loud sound do not (like 

 those of light) move parallel to each other from the surface 

 of a concave reflector, but constantly diverge laterally on all 

 sides; and although at first they are more intense in the axis 

 of the reflector they finally spread out so as to encompass 

 the whole horizon, thus rendering the use of reflectors to 

 enforce sound — of little value in fog-signals. 



2d. That the effect of wind in increasing or diminishing 

 sound is not confined to currents of air at the surface of the 

 earth, but that those of higher strata are also efficient in 

 varying its transmission. 



3d. That although sound is generally heard farther with 

 the wind than against it, yet in some instances the reverse is 

 remarkably the case, especially in one locality, in which the 

 sound is frequently heard against a north-east snow-storm 

 more distinctly than when the wind is in an opposite direc- 

 tion. This anomaly was referred to the action of an upper 

 current in an opposite direction to that at the earth, such a 

 current being known to exist in the case of north-east storms 

 on our coast. But in what manner the action of the wind in- 

 creased or diminished the audibility of sound was a problem 

 not solved. It could not be due, as might be thought at first 

 sight, to the acceleration of the sonorous impulse by the 

 addition of the velocity of the wind to that of sound, on the 



