Jan. 29, 1874J 



NATURE 



251 



THE ACObSTIC TRANSPARENCY AND 



OPACITY OF THE ATMOSPHERE* 



T^IIE cloud produced by the puff of a locomotive can ob- 



literate the noonday sun ; it is not therefore surprising 



that in dense fogs our most powerful coast lights, including even 



tile electric light, become useless to the mariner. 



A disastrous loss of life and property is the consequence. 

 During the last ten years, for example, the number of total 

 wrecks on the coasts of the United Kingdom, which were re- 

 ported to have been caused by fog and thick weather, amounted, 

 I am informed, to 273 vessels. 



C'f late years various efforts have been made, both on our own 

 coa.sts and on the American seaboard, wliere trade is more eager 

 and fogs more frequent than they are here, to furnish warning 

 and guidance to ships by means of sound signals of great power 

 established along the coast. Regarding the performance of such 

 signals, the most conflicting evidence exists ; and no investigation 

 has been hitheito instituted sufficiently exhaustive to remove the 

 uncertainty. 



The problem has occupied for some time the attention of the 

 Elder Brethren of the Trinity House ; and soon after my return 

 from America they requested me, as their official adviser in 

 scientific matters, to superintend an investigation of the entire 

 subject. They had appointed a committee under whose auspices 

 two st.itions had been established at the South Foreland. I 

 entered upon the inquiry with such ardour as I could derive 

 from a sense of duty, rather than from the pleasure of hope, for 

 I knew it would be long and difhcult, and that I was at the mercy 

 of a medium, the earth's atmosphere, \\hich could not be put into 

 the witness-box and cross-examined scientifically. The experi- 

 menter can usually impose his own conditions upon Nature, 

 and force her to reply. In the present case we were forced to 

 accept the conditions which Nature imposed. 



Nevertheless, if the student only holds on faithfully to any 

 natural problem, intending his mind upon it, and not falling into 

 hasty despair, he is sure to be rewarded in the end; and after a time 

 results, important not only in a practical but in a |mrely scientific 

 point of view, appeared to grow out of the investigation. I men- 

 tioned this to the Deputy Master of the Trinity House, saying 

 that I thought such results might, without impropriety, be 

 communicated to the Roy.al .Society and the Royal Institution. 

 His response was ]irompt and cordial, and he was seconded 

 by his colleagues in this response. They gave not only the re. 

 quested permission (which on various pleas they might have with- 

 held), but they have aided me in every way in the preparation 

 of this discourse. 



I would add that the Elder Brethren themselves have had a 

 large share in the executive portion of this investigation, and 

 whatever success has attended the inquiry is in a greit measure 

 due to the cheerful promptness and thoroughness with which my 

 wishes and suggestions were carried out by the gentlemen with 

 whom I had the honour to act. It is not necessary to mention 

 names when all have been so sympathetic and so helpful, but 



I should like to refer to a few gentlemen on the working staff of 

 the Trinity House, who have aided me with all assiduity and 

 all zeal. They are the able Trinity House engineer, Mr. Doug- 

 lass, his assistant engineer, Mr. Ayres, and Mr. Price Edwards, 

 the private secretary of the Deputy Master of the Trinity House. 



On Monday, May 19, the experiments began. The instruments 

 employed had been previously mounted at the top and bottom of 

 the South Foreland Cliff. They were two brass trumpets, or horn?, 



II ft. 2 in. long, 2 in. in diameter at the mouth-piece, opening 

 <ii;t at the other end to a diameter of 22 in. They were juo- 

 \ : lid with vibrating steel reeds, 9 in. long, 2 in. wide, and { in. 

 thick, and were sounded by air of 18 lbs. pressure. They were 

 mounted vertically on the reservoir of compressed air ; but 

 within about 2 ft. of their extremities the)' were bent at a right 

 angle, so as to present their mouths to the sea. These horns 

 were constructed by Mr. Holmes. There were also two whistles 

 shaped like those in use on locomotives, one 6 in. in diameter, 

 sounded by air of 18 lbs. pressure ; the other constructed by Mr. 

 Eaily of Manchester, 12 in. in diameter and sounded by steam 

 of 64 lbs. pressure. 



We embarked on the steamer Irene, and placed ourselves 

 abreast of the signal-station, halting at a distance of half a mile 

 from it. The wind was strong, the sea rough. The superiority 

 of the trumpets to the whistles was very marked, and I may 



say continued marked throughout. Their sound was exceedingly 

 fine and powerlul. At I mile's distance their sound was clear and 

 strong ; at 2 miles they were heard distinctly, though not loudly. 

 The whistles were also heard, but as fog-.'ignals they had be- 

 come useless. At 3 miles the horns I'ecame also useless. It 

 required great attention to hear them distinctly. At a distance 

 of 4 miles, with the paddles stopped, we listened long and atten- 

 tively, but heard nothing. 



On May 20, at 3 miles' distance, the steam whistle was not 

 at all heard, the horns but faintly.- At 4 miles' distance, the air 

 being very light, the sea calm, and the circumstances generally 

 to all appearances highly favourable, we halted and lis>ened. 

 The horns were so heard as to render it unmistakeable that a 

 sound was there. At 4-8 miles the sounds were faintly heard ; 

 at 5 miles an occasional murmur reached us. At 6 miles the 

 faint hum of a horn was wafted to us at intervals A little farther 

 out, though local noises were absent, and though we listened 

 with stretched attention, we heard nothing. 



This position, clearly beyond the range of whistles and 

 trumpets, was chosen with the view of making a decisive com- 

 parative experiment between horns and guns as instruments for 

 fog-signalling. Through the courtesy of General Sir A. Hors- 

 ford we were enabled to carry out this comparison. At 12-30 

 precisely the puff of an iS-pounder, with a 3-lb. charge, was 

 seen at Dover Castle, which was about a mile farther off than the 

 South Foreland. Thirty-six seconds afterwards the loud report 

 of the gun was heard, its complete superiority over the trumpets 

 being thus to all appearance demonstrated. 



We clinched this observation by ste.aming out to a distance ot 

 SJ miles, where the report of a second gun was well heard. At 

 10 miles the report of the gun was heard Ly some and not by 

 others. At 97 miles a fourth report was heard by all 

 observers. 



There was nothing, far as I am aware o'', in our knowledge 

 of the transmission of sound through the atmosphere, to invali- 

 date the founding upon these experiments of the general conclu- 

 sion that, as a fog-signal, the gun possessed a clear mastery over 

 the horns. No observation, to my knowledge, had ever been 

 made to show that a sound once predominant would not always 

 be predominant ; or that the atmosphere on different days would 

 show preferences to different sounds. A complete reversal of the 

 foregoing conclusion was therefore not to be anticipated ; still, 

 on many subsequent occasions, it was completely reversed. 



On June 2 the maximu-n range, at first only 3 miles, after- 

 wards ran up to about 6 miles. 



Optically June 3 was not at all a promising day ; the clouds 

 were dark and threatening ; and the air filled with a faint haze, 

 nevertheless the horns were fairly audible at 9 miles. An ex- 

 ceedingly heavy rain-shower approached us at a galloping speed. 



* Royal Institutioa, Friday evening Discour; 

 LI,.D.,F.R.S. J.-in. 16. 



Iiy I'rof. Tyndall, D.CL. The sounds were not sensibly impaired during the continuance 

 I of the rain. This state of the atmosphere, according to hitherto 



