187S] WRITINGS OF JOSEPH HENRY. 451 



miles, while at another time the sound of the same instru- 

 ment cannot be heard at more than a fifth of the same dis- 

 tance; fourth, the circumstance that while the sound is 

 heard generally farther with the wind than against it, in 

 some instances the reverse is the case; fifth, the sudden loss 

 of sound in passing from one locality to another in the same 

 vicinity, the distance from the source of the sound being the 

 same. 



The first four of these phenomena find a ready explana- 

 tion in the hypothesis adopted, b}^ supposing an increase or 

 diminution in the relative velocity of the currents of wind 

 in the upper and lower strata of air. The fifth is explained 

 — either by an irregular twisting of the sound-beam, (as 

 above suggested,) or b}' the inter-position of an obstacle 

 which casts a sound-shadow — disappearing at a given dis- 

 tance by the convergence of the rays on each side of the 

 obstacle into what would be an optical shadow. 



Accounts of these investigations were presented from time 

 to time to the Light-House Board and to the Philosophical 

 Society of Washington in 1872. Subsequently a series of 

 investigations on the same subject was instituted in England 

 by the Elder Brethren of the Trinity House under the direc- 

 tion of their scientific adviser, the eminent physicist. Dr. 

 Tyndall. While in the latter investigations various abnor- 

 mal phenomena similar in most instances to those we have 

 mentioned were observed, they were referred by Dr. Tyndall 

 to an entirel}' different cause, viz, to the existence of acoustic 

 clouds, consisting of portions of the atmosphere in a floccu- 

 lent or mottled condition, due to the unequal distribution of 

 heat and moisture, which absorbing and reflecting the sound, 

 produce an atmosphere of acoustic opacity. While we do 

 not deny the possible existence of such a condition of the 

 atmosphere, we think it insufficient to account for all the 

 phenomena in question, and believe that a more general and 

 efficient cause is that of the wi7id, in accordance with the 

 hypothesis of Professor Stokes. 



We regret to differ in opinion from Dr. Tyndall, and have 

 published our dissent from his views in no spirit of captious 



