444 WRITINGS OF JOSEPH HENRY. [1874 



were unusually favorable, the weather continuing the same 

 during the two days, and the change of the wind also tak- 

 ing place at nearly the same hour. 



The fact thus established seems entirely incompatible with 

 the supposition that the diminution in the sound is princi- 

 pally caused by a want of homogeneity in the constitution of 

 the atmosphere, since this would operate to absorb sound 

 equally in both directions. 



In May, 1873, Professor Tyndall commenced a series of 

 investigations on the subject of the transmission of sound, 

 under the auspices of the Trinity House, of England, in 

 which whistles, trumpets, guns, and a siren were used ; the 

 last-named instrument having been lent by the Light-House 

 Board of the United States to the Trinity House for the pur- 

 pose of the experiments in question. The results of these 

 investigations were in most respects similar to those which 

 we had previously obtained. In regard to the efficiency of 

 the instruments, the same order was determined which has 

 been given in this report, namely the siren, the trumpet, and 

 the whistle. Professor Tyndall's opinion as to the efficiency 

 of the siren may be gathered from the following remarks. 

 Speaking of the obstruction of sound in its application as a 

 fog-signal, he says, " There is but one solution of this diffi- 

 culty, which is to make the source of sound so powerful as 

 to be able to endure loss and still retain sufficient residue 

 for transmission. Of all the instruments hitherto examined 

 by us the siren comes nearest to the fulfillment of this con- 

 dition, and its establishment on our coasts will in my opin- 

 ion prove an incalculable boon to the mariner." Professor 

 Tyndall arrived at the conclusions which the information 

 we had collected tended to establish, that the existence of 

 fog however dense does not materially interfere with the 

 propagation of sound, and also that sound is generally 

 heard farther with the wind than against it, although the 

 variation of the intensity of the sound is not in all cases in 

 proportion to the velocity of the wind. The result of his 

 investigations in regard to the pitch of sound was also sim- 

 ilar to those we have given; and indeed all the facts which 



