ON THE MOTIONS OF SOUND. 421 



views have been already complied with ; but if they be not and I am strongly 

 inclined to think that they are not tlien I would submit that it behooves us to 

 have the best, and to aim at making the South Foreland, both as regards light 

 and sound, a station not excelled by any other in the world." 



On this score it gives me pleasure to say that I never had a diflS- 

 culty with the Elder Brethren. They agreed with me ; and two power- 

 ful steam-whistles, the one from Canada, the other fi-om the United 

 States, together with the steam-siren also an American instrument 

 were in due time m.ounted at the South Foreland. It will be seen, in 

 Chapter VIL, that my strongest recommendation applies to an instru- 

 ment for which we are indebted to the United States. 



In presence of these facts, it will hai'dly be assumed that I wish to 

 withhold from the Lighthouse Board of Washington any credit that 

 they may fairly claim. My desire is to be strictly just ; and this de- 

 sire compels me to express the opinion that their report fails to es- 

 tablish the inordinate claim made in its first paragraph. It contains 

 observations, but contradictory observations; while, as regards the 

 establishment of any principle which should reconcile the conflicting 

 results, it leaves our condition unimproved. 



But I willingly turn aside from the discussion of " claims " to the 

 discussion of science. Inserted, as a kind of intrusive element, into 

 the Keport of Prof. Henry, is a second Report by General Duane, 

 founded on an extensive series of observations made by him in 1870 

 and 1871. After stating with distinctness the points requiring deci- 

 sion, the general makes the following remarks : 



" Before giving the results of these experiments, some facts will be stated 

 which will explain the difficulties of determining the power of a fog-signal. 



" There are six steam fog-whistles on the coast of if aine : these have been fre- 

 quently heard at a distance of twenty miles, and as frequently cannot be heard 

 at the distance of two miles, and this with no perceptible difference in the state 

 of the atmosphere, 



" The signal is often heard at a great distance in one direction, while in an- 

 other it will be scarcely audible at the distance of a mile. This is not the effect 

 of wind, as the signal is frequently heard much farther against the wind than 

 with it.' for example, the whistle on Cape Elizabeth can always be distinctly 

 heard in Portland, a distance of nine miles, during a heavy northeast snow- 

 storm, the wind blowing a gale directly from Portland toward the whistle.^ 



" The most perplexing difficulties, however, arise from the fact that the sig- 

 nal often appears to be surrounded by a belt, varying in radius from one to one 

 and a half mile, from which the sound appears to be entirely absent. Thus, in 

 moving directly from a station the sound is audible for the distance of a mile, is 

 then lost for about the same distance, after which it is again distinctly heard for 



' That is to say, homogeneous air with an opposing wind is frequently more favorable 

 to sound than non-homogeneous air with a faToring wind. We made the same experience 

 at the South Foreland. J. T. 



* Had this observation been published, it could only have given me pleasure to refer 

 to it iu my recent writings. It is a stril^ing confirmation of my observations on the Mer 

 de Glace in 1859. 



