252 



NA TURE 



\7an. 29, 1874 



expressed opinions, should have deadened the sound. It rather 

 aided the sound, and this added to my perplexity. 



On June 10 the maximum ranije was 9 miles. An extraordi- 

 nary sinking of tlie sound was, however, noticed on the Dover 

 side of the Foreland. At a mile's distance from the station the 

 sounds rapidly fell. Suq>rised at the suddenness of the effect, 

 and thinking it might be due to some peculiarity of the horns, at 

 2 miles' distance I signalled for the guns. With a 3 lb. charge 

 not one of them was heard. 



On June 1 1 we steamed towards the South Sound Head light- 

 ship. At the distance of i\ miles, and even at 2 miles and less 

 from the station, the sounds were not so strong as at 35 rrriles. 

 We steamed abreast of the station and on to the line joining the 

 South Foreland to the end of the Admiralty Pier. At three- 

 quarters of a mile from the station the sound fell, and a little 

 farther on was scarcely audible. This weakening of the sound 

 between the pier and the Foreland was invariable. This needs 

 a word of explanation. The fall of the sound is rot caused 

 directly by an acoustic shadow, for it occurs when the instru- 

 ments are in view, but the limit of an acoustic shadow is close 

 at hand. A little within the line joining the Foreland and the 

 pier end, the instruments are cut off by a projection of the cliff 

 near the station ; all the sea space between this limit and the 

 cliff under Dover Castle is in the shadow. Into this, however, 

 the direct waves diverge, and lose intensity by their divei'gence, 

 the portion of the wave nearest the shadow suffering most. To 

 this must be added the effect of interference. 



On June 25 the range was 5J miles. On June 26 the range 

 was 10 miles. The former day the wind was in the direction of 

 the sound ; on the latter the wind was opposed. Plainly 

 there must be something besides the wind which determines the 

 sound-range. This something was now the object of search. 



Is it the clearness of the atmosphere ? All previous writers 

 have extolled a clear atmosphere as best for sound ; but on July iS 

 we steamed out to a distance of 10 miles and heard sounds, the 

 white cliffs of the Foreland being at the same time entirely 

 hidden in thick haze. Nay, more : we spoke the Tn'Uvr tender 

 on its way from the I'ariie lightship, and took the master of the 

 Vanie on board. He reported that the sounds had been heard 

 at the lightship, though it is 12J miles fr-om the P'oreland. It 

 was, moreover, dead to windward of the Foreland, so that both 

 haze and wind were then in opposition ; still the sound ranged 

 at least twice as far as it had done on days when neither haze nor 

 wind was there to interfere with the sound. 



On July 2, a sudden acoustic darkness, if I may use the term, 

 settled upon the atmosphere. The range was only 4 miles. The 

 magnitude of the fluctuations, from 3.^ to 12-4' miles, observed 

 up to this date, was striking : but I was unable to fix upon any 

 meteorological element that coul 1 be held accountable for them. 

 The wind, the clearness of the air", the liaromcter, the thermo- 

 meter, the hygrometer, gave me no help. All was perplexity. 

 I longed for light, but saw little prospect of obtaining it. 



July 3 was a lovely morning : the sky was of a stainless 

 blue, the air calm, and the sea smooth. I thought we should 

 be able to hear a long way off. \Ve steamed beyond the pier 

 end and listened. The steam clouds were there, showing the 

 whistles to be active ; the smoke puffs were there, attesting the 

 activity of the guns. Nothing was heard. We went nearer ; 

 but at two miles horns ami whistles and guns were equally in- 

 avrdible. This however being near the limit of the sound shadow, I 

 thought that might have something to do with the effect, so we 

 steamed right in front of the station, and halted at 34 miles from 

 it. Not a ripple nor a breath of air disturi^ed the stillness 0:1 

 board, but we heard nothing. There were tlie steam-puffs from 

 the whistles, and we knew that letween every two pufTs the 

 horn sounds were embraced, but we heard nothing. We sig- 

 nalled for the guns ; there were the smoke puffs apparently close at 

 hand, but not the slightest sound. It was mere dumb show on 

 the Foreland. We steamed in to 3 miles, halted, and listened 

 with all attention. Neither the horns nor the whistles sent u-; 

 the slightest hint of a sound. The guns wtre again signalled 

 for ; five of them were fired, some elevated, some fired point 

 blank at us. Not one of them was heai'd. We steamed in 10 

 two miles, and had the guns ayain fired: the howitzer and 

 mortar with 3 lb. charges yielded the faintest thud ; and the 

 l8-pounder was quite unheari). 



In the presence of these facts I stood amazed and confounded, 

 for it had been assumed and affirmed by distinguished men who 

 had given special attention to this subject, that a clear, calm 

 atmosphere was the best vehicle of sound : optical clearness and 



acoustic clearness were supposed to go hand in hand : indeed, it 

 had been proposed to make the one a measure of the other. 

 But here was a day perfectly optically clear, proving itself to be 

 a day of acoustic darkness almost impenetrable. I was driven 

 slowly to the conclusion that all I had read upon this subject 

 was wrong, and that for 165 years, namely since 1708, when 

 Dr. Derham published his celebrated paper on this subject, suc- 

 ceeding generations of scientific men had gone on repeating the 

 same errors. This knowledge, however, did not help me much. 

 The problem was still there challenging solution. 



I ventured, two or three years ago, to say something regarding 

 the function of the Imagination in Science, and notwithstanding 

 the care that I took to define and illustrate its real province, 

 many persons, amongst whom were one or two able men, 

 deemed me lOose and illogical ; in fact, merely poetic, when I 

 referred to the imagination. The history of science, however, 

 numbers many men of strong poetic temperament, who, in the 

 presence of a scientific problem, became as cold and clear 

 as the light of stars. Look at these two pieces of polished steel. 

 Have you a sense, or the rudiment of a sense, to distinguish the 

 inner condition of the one from that of the other ? And yet 

 they dilTer materially, for one is a magnet, che other not. What 

 enabled that noble philosopher, and pure and elevated character, 

 Ampere, to surround the atoms of such a magnet with channels 

 in which electric currents ceaselessly run, and to deduce from 

 these pictured currents all the phenomena of ordinary magnet- 

 ism ? What enabled Faraday to visualise his lines of force, to 

 follow them through magnets and through space until his mental 

 picture became a guide to discoveries which have rendered this 

 place immortal ? What but imagination ? I have reason to 

 know but too well the fantastic, and even scandalous use that is 

 made of the faculty when it is divorced from the disciplined 

 understanding and handed over to the undisciplined passions and 

 emotions. But this is not the scientific use of the imagination. 



And now to return. Figure yourself on the deck of the Jiene, 

 with the invisible air stretching between you and the South 

 Foreland, knowing that it contained something which stifled the 

 sound, but not knowing what that something is. Vour senses 

 are not of the least use to you ; you are unable to see, or hear, 

 or feel, or taste, or smell the object of your search ; nor could 

 all the philosophical instruments in the world, as it now is, 

 render you the least assistance. You cannot take a single step 

 towards the solution without the formation of a mental image, 

 in other words, without the exercise of the imagination. Let 

 me unfold my own e.xact course of thought and action. 



Sulphur in homogeneous crystals is exceedingly transparent 

 to radiant heat, whereas the ordinary brimstone of commerce is 

 highly impervious to it. Why ? Because the brimstone of com- 

 merce does not possess the molecular continuity of the crystal, 

 but is a mere aggregate of minute grains not in pefect optical 

 contact with each other. When this is the case, a portion of the 

 heat is always reflected on entering and quitting a grain. Hence 

 when the grains are minute and numerous, this reflection is so 

 often repeated that the heat is entirely wasted before it can 

 plunge to any depth in the substance. A snowball is opaque to 

 liijht for the same reason. It is not optically continuous ice, but 

 an aggregate of grains of ice, and the light which falls upon the 

 snow being reflected at the limiting surfaces of the snow 

 granules, fails to penetrate the snow to any depth. Thus by the 

 mixture of air and ice, two transparent substances, we produce a 

 substance as impervious to light .as a really opaque one. The 

 same remark applies to foam, to clouds, to common salt, in- 

 deed to all transparent substances in powder. They are all 

 impervious to light, not through the real absorption or extinction 

 of the light, but through internal rellection. 



Humboldt, in his observations at the Falls of the Orinoco, is 

 known to have applied these principle.'. He found the noise of 

 the Falls three times louder by night than by day, though in that 

 ref;ion the night, through be.asts and insects, is far noisier than 

 the day. The plain between him and the Falls consisted of 

 spaces of grass ami rock intermingled. In the heat of the day 

 he found the temperature of the rock to be 30° higher than that 

 of the grass. Over every heated rock, he concluded, rose a 

 column of air rarefied by the hcit, and he ascribed the deadening 

 of the sound to the reflCLtions which it endured at ihe limiting 

 surfaces of the rarer and denser air. This philosophical expla- 

 nation made it generally known that a non-homogeneous atmo- 

 sphere is unfavourable to the transmission of sound 



But what on July 3, over a calm sea, where neither 

 rocks nor grass existed, could so destroy the homogeneity 



