41 8 THE POPULAR SCIENCE MONTHLY. 



it necessary to reply, believing that the grounds of it would disappear 

 in presence of the full account. The only opinion to which I thought 

 it right to defer was to some extent a private one, communicated to 

 me by Prof. Stokes. He considered that I had, in some cases, ascribed 

 too exclusive an influence to the mixed currents of aqueous vapor and 

 air, to the neglect of differences of temperature. That difierences of 

 temperature, when they come into play, are an efficient cause of acous- 

 tic opacity, I never doubted. In fact, aerial reflection arising from 

 this cause is, in the present inquiry, for the first time made the subject 

 of experimental demonstration. What the relative potency of differ- 

 ences of temperature and differences due to aqueous vapor, in the 

 cases under consideration, may be, I do not venture to state ; but, as 

 both are active, I have, in Chapter VII., referred to them jointly as 

 concerned in the production of those " acoustic clouds " to which the 

 stoppage of sound in the atmosphere is for the most part due. 



Subsequently, however, to the publication of the full investigation, 

 another criticism appeared, to which, in consideration of its source, I 

 would willingly pay all respect and attention. In this criticism, which 

 reached me first through the columns of an American newspaper, dif- 

 ferences in the amounts of aqueous vapor, and differences of tempera- 

 ture, are alike denied efficiency as causes of acoustic opacity. At a 

 meeting of the Philosophical Society of Washington, the emphatic opin- 

 ion had, it was stated, been expressed that I was wrong in ascribing 

 the opacity of the atmosphere to its flocculence, the really efficient cause 

 being refraction. This view appeared to me so obviously mistaken that 

 I assumed, for a time, the incori-ectness of the newspaper account. 



Recently, however, I have been favored with the " Report of the 

 United States Lighthouse Board for 1874," in which the account just 

 referred to is corroborated. A brief reference to this report will here 

 suffice. Major Elliott, the accomplished officer and gentleman referred 

 to at page 261, had published a record of his visit of inspection to this 

 country, in which he spoke, with a pei'fectly enlightened appreciation 

 of the facts, of the differences between our system of lighthouse illumi- 

 nation and that of the United States. He also embodied in his report 

 some account of the investigation on fog-signals, the initiation of which 

 he had M'itnessed, and indeed aided, at the South Foreland. 



On this able report of their own officer the Lighthouse Board at 

 Wasliington make the following remark : " Although this account is 

 interesting in itself and to the public generally, yet, being addi'essed 

 to the Lighthouse Board of the United States, it would tend to convey 

 the idea that the facts which it states were new to the Board, and that 

 the latter had obtained no results of a similar kind ; while a reference 

 to the Appendix to this Report * will show that the researches of our 



' It will be borne iu mind that the Washington Appendix waa published nearly a year 

 aftfir mj Eeport to the Trinity House. 



