268 



NATURE 



[Feb. 5, 1874 



235 feet above us, the sea was smooth and clear of ships, the 

 atmosphere was without a cloud, and there was no object in 

 sight which could possibly produce the observed effect. From 

 the perfectly transparent air the echoes came, at first with a 

 strength apparently but litile less than that of the direct sound, 

 and then dying gradually and continuously away. The remark 

 of my companion, Mr. Edwards, was : " Beyond saying that 

 the echoes seemed to come from the expanse of ocean, it did not 

 appear possible to indicate any more definite point of re- 

 flection." Indeed, no sucli point was to be seen; the echoes 

 reached us as if by magic, from absolutely invisible walls. 

 Arago's notion that clouds are necessary to produce atmospheric 

 echoes is therefore untenable. 



The reflection fiom aerial surfaces has never been experi- 

 mentally demonstrated. It is wholly a matter of inference, and 

 I wished very much lo reduce it to demonstration. I made one 

 or two rough experiments on the transmission of sound through 

 a series of flames ; and no doubt by proper arrangement such 

 experiments might be made successful. I then thought that 

 alternate layers of carbonic acid and coal ga=, the one rising by 

 its lightness, the other falling by its weight, would supply a 

 heterogeneous medium suitable for the demonstration. To my 

 assistant, Mr. Cottrell, who possesses in an eminent degree the 

 skill of devising apj^ara'us, I communicated this idea, leaving 

 the realisation of it wholly to him, and he has carried it out in 

 the most admirable manner. (For a sketch and description of 

 the apparatus, see previous page). 



During my recent visit to the United States I accompanied 

 General Woodruff, the engineer in charge of two of the light- 

 house districts, to the establishments at Staten Island and 

 Sandy Hook, with the express intention of observing the per- 

 formance of the steam-syren, which, under the auspices of Prof. 

 Henry, has been introduced into the lighthouse system of the 

 United States. Such experiments as were possible to make 

 under the circumstances were made, and I carried home with 

 me a somewhat vivid remembrance of the mechanical effect of 

 the sound of the steam syren upon my ears and body generally. 

 This I considered to be greater than the similar effect produced 

 by the horns of Mr. Holmes ; hence the desire, on my part, to 

 see the syren tried at the South Foreland. The formal ex- 

 pression of this desire was anticipated t.y the Elder Brethren, 

 while their wishes were in turn anticipated by ihe ci.urteous 

 kindness of the Lighthouse Board at Washington. Informed 

 by Major Elliott that our experiments had begun, the Board for- 

 warded to the Corporation, for trial, the noble instrument now 

 mounted at the South Foreland. The principle of the syren is 

 easily understood. A musical sound is produced when the 

 tympanic membrane is struck periodically with sufficient rapidity. 

 The production of these tympanic shocks by puffs of air was 

 first realised by Dr. Robison. But the syren itself is the inven- 

 tion of Cagniard de la Tour. He employed a box with a per- 

 forated lid, and above the lid a similarly perforated disc, capable 

 of rotation. The perforations were oblique, so that when wind 

 was driven through the disc was set in motion. When the per- 

 forations coincided a puff escaped, when they did not coincide 

 the current of air was cut off. The regular succession of impulses 

 thus imparted to the air produce a musical note. Even in i;s 

 small form, the instrument is capable of producing sounds of 

 great intensity. The syren has been improved upon by Dove, 

 and notably developed by Helmlioltz. 



In the steam syren patented by Mr. Brown of New York, a 

 fixed disc and a rotatory disc are also employed, radial slits 

 being cut in both discs instead of circular apertures. One disc 

 is fixed across the throat of a trumpet-shaped tube, 1 61 ft. long, 

 5in. diameter where the disc crosses it, and gradually opening 

 out till at the other extremity it reaches a diameter of 2 ft. 3 in. 

 Behind the fixed disc is the rotating one, which is driven Viy 

 separate mechanism. The trumpet is mounted on a boiler. In 

 our experiments steam of 70 lbs. pressure has for the most part 

 been employed. Just as in the air-syren, when the radial slits of 

 the two discs coincide, a putf of steam escapes. Sound-waves of 

 great intensity are thus sent through the air ; the pitch of the 

 note produced depending on the rapidity with which the puffs 

 succeed each other ; in other words, upon the velocity of 

 rotation. 



On October S I remained some time at the Foreland, listening 

 to the echoes. Of tlie horn-echoes I have already spoken : those 

 of the syren were still more extraordinary. Like the others they 

 were perfectly continuous, and faded as if into the distance gra- 



dually away. The single sound seemed rendered complex and mul- 

 tiiudinims by its echoes, which resembled a bar.d of trumpeters 

 first responding close at hand, and then retreating rapidly from 

 us towards the coast of France. The syren echoes had eleven 

 seconds duration, those of the horn eight seconds. With sounds 

 of the same pitch the duration of the echo might be taken as a 

 measure of the ])eiietrative power of the sound. 



I moved away from the station so as to lower the power of 

 the direct sound. This was done by dropping into the sound- 

 shadow behind an a'ljacent eminence. The echoes heard thus 

 \vere still more wonderful than before. In the case of the syren, 

 moreover, the reinforcement of the direct sound by the echo was 

 distinct. One second after the commencement of the syren blast, 

 the echo struck in as a new sound. This first echo, therefore, 

 must have been flung back by a body of air not more than 600 

 or 700 ft. in thickness. 



There appears to ba a direct connection between the duration 

 of the echoes and the distance penetrated by the sound. On 

 October 17 the perfect clearness of the afternoon caused me to 

 choose it for the examination of the echoes. The echoes of that 

 day, when our transmitted sound reached its maximum, exceeded 

 in duration those of all other days. We heard the syren fifteen 

 miles oft'. On the close of the day we found its echoes fourteen 

 to fifteen seconds in duration, this long duration indicating the 

 distance from which they were thrown back. 



The visual clearness of the atmosphere on the morning of Oct. 8, 

 was very great, the coast of France was very plainlyseen, theGrisnez 

 lighthouse, and the monument and cathedral of Boulogne, were 

 distinctly visible to the naked eye. At }\ miles from the station, 

 the horn was heard feebly, the syren clearly. At 2.30 P.M., a 

 densely black scowl overspread the heavens to the W. S.W. At 

 this hour, the distance being 6 miles, the horn was heard very . 

 feebly, the syren more distinctly, all being hushed on board 

 during the observations. A squall now approached us from the 

 west. In the Alps, or elsewhere, I have rarely seen the heavens 

 blacker. Vast cumuli floated in the N.E. and S.E. ; vast 

 streamers of rain were seen descending W.N. W. ; huge scrolls 

 of cloud to the N. 



At 7 miles distance the syren was not strong, and the hom 

 was very feeble. 



The heavy rain at length reached u=, but although it was fall- 

 ing all the way between us and the Foreland, the sound, instead 

 of being deadened, rose perceptibly in power. Hail was now 

 added to the rain, and the shower readied a tropical violence. 

 We stopped. In the midst of this furious squall both the horn 

 and the syren were distinctly heard, and as the shower lightened, 

 thus lessening the local pattering, the sounds so rose in power 

 that we heard them at a distance of 7I miles distinctly louder 

 than they had been heard through the rainless atmosphere at 5 

 miles. This observation is entirely opposed to the statement of 

 Deiham, which has been repeated by all writers since his time, 

 regarding the stifling influence of falling rain upon sound. But 

 it harmonises perfectly with our experience of July 3, which 

 proved water in a state of vapour so mixed with air as to form 

 non-homogeneous parcels, to be a most potent influence as re- 

 gards the stoppage of sound. Prior to the violent shower, the air 

 had been in this flocculent condition, but the descent of the rain 

 and hail restored in part the homogeneity of the atmosphere, and 

 augmented its transinissive power. There may be states of the 

 atmosphere associate'! with rain unfavourable to sound, but to 

 rain itself I have never been able to trace the slightest deadening 

 effect. 



The observations continued till November 25. Up to that 

 date we had no fog, but the experience of July I and of October 

 30, entirely destroy the notion that optical transparency and 

 acoustic transparency go hand-in-hand. Both were days of haze 

 sufficiently thick to hide the cliffs of the Foreland, but on the 

 former the sounds reached 125, and on the latter lij- miles. 



Keflection from the particles of fog and haze has been hitherto 

 held to blot out sound. The late dense fog in London enabled 

 experiments to be made which entirely controvert this conclusion. 

 On December 10 I made some experiments over the Serpentine. 

 Thefog was very dense. Mr. Cottcrell stood on the walk below the 

 south-west end of tf.e bridge dividing Hyde Park from Kensing- 

 ton Gardens, while I went to the eastern end of the Serpentine. 

 He blew a dog-whistle, and an organ-pipe sounding Mi.,, which 

 corresponds to 380 waves a second. I heard botti distinctly. I 

 then changed places with him, and listening attentively at the 

 fridge, heard for a time the distinct blast of the whistle only. 

 The organ-pipe at length sent its deeper note to me across the 



