RESEARCHES IN SOUND. 509 



« 

 tlie water, and tluis producing- a current from tlie latter to the former, 

 while the wind observed in the moruiug from the west was the land- 

 wind due to the cooling of the latter. 



In the morning the wind was blowing from the west both in the 

 higher strata and at the surface of the earth, and in this condition the 

 sound was heard farther with the wind than against it. 



The wind at the surface about midday gradually ceased, and shortly 

 afterward sprang up from an east direction; in this condition the sound, 

 with the wind at the surface, was heard at a greater distance. This is 

 also in strict conformity with the theory of a change in the form of the 

 sound-wave, as in the latter case the lower portion would be retarded, 

 while the upper portion of the wave would be carried forward with the 

 same velocity, and hence the sound would be thrown down on the ear 

 of the observer. To explain the result of the third trial of the second 

 day, we have only to suppose that the influence of the upper current 

 was less than that of the lower. The conditions for these observations 

 were unusually favorable, the weather continuing the same during the 

 two days, and the change of the wind also taking place at nearly the 

 same hour. 



The fact thus established is entirely incompatible with the supposition 

 that the diminution in the sound is principally 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 

 purpose of the experiments in question. The results of these investi- 

 gations 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 bo 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 difficulty, which is to make 

 the source of sound so powerful as to be able to endiu-e loss and still 

 retain sufficient residue for transmission. Of all the iustrnments hitherto 

 examined by us the siren comes nearest to the fulfillment of this condi- 

 tion, and its establishment upon our coasts will, in my opinion, 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 variatiou of the in- 



