486 WRITINGS OF JOSEPH HENRY. [1875 



establish the true theory of the abnormal phenomena of 

 sound ; an object (independent of its scientific interest) of 

 much practical importance in its application to fog-signals. 

 Although the observations were not as perfect as we could 

 wish in many respects, (from want of certain appliances,) they 

 are yet sufficient we think to establish principles of much 

 practical value. For example, if the mariner in approach- 

 ing a fog-signal — while the wind is blowing against the sound* 

 fails to perceive it on deck, he will probably hear it by 

 ascending to the mast-head ; or in case a sound from a given 

 station is constantly obscured in a certain direction, while it 

 is audible in adjacent directions, we may attribute it to 

 a sound-shadow produced by some interposed object. If 

 again the obscuration of sound in a given direction is only 

 observed during a wind moving against the sound, the cause 

 will probably be found in a lateral refraction, due to the 

 retardation of the current of wind against a perpendicular 

 wall or cliff, as in the case observed at Block Island August 

 19, The subject is indeed one of great complexity, and 

 requires further investigation; but the results thus far ob- 

 tained may be considered as furnishing the preliminary 

 data on which to found more precise observations. These 

 should be made with the aid of a number of steamers simul- 

 taneously employed, each furnished with anemometers and 

 balloons for determining with more accuracy the direction 

 and velocity of the wind. 



We hope to renew the investigations during next summer, 

 and in view of this, have directed that in the meantime the 

 light-keepers at Block Island and at Point Judith shall con- 

 tinue to sound their sirens a certain length of time every 

 Monday, noting the direction and velocit^^ of the wind, the 

 temperature and pressure of the air, and the audibility of 

 the sound as it comes reciprocally from each instrument. 



It is shown from the results thus far obtained from these 

 reciprocal observations that sound is occasionally heard 

 more distinctly against the wind than in a contrar}' direc- 

 tion. We think however that these instances are generally 

 followed by a change in the direction of the wind at the sur- 

 face of the earth 



