Digitale Ausgabe

THE increase in the intensity of sound during thenight, is a problem which is not resolved in any of theworks on natural philosophy. I shall endeavour to givea solution derived from the latest researches on the theoryof sonorous undulations; but before treating of the causeof this phenomenon, I shall mention the circumstancesunder which it takes place. It has been remarked from the earliest antiquity thatthe intensity of sound increases during the night. Aris-totle has spoken of it in his Problems, and Plutarch inhis Dialogues. It is only the increase of sound when theair is calm that we are now treating of, not that whichaccompanies a change of wind in the night, and which ismodified by the relation that exists between the directionof the wind, and that of the sonorous ray. Under thesame latitude (between the tropics for example), thenightly increase of sound has appeared to me greater onthe plains than on the heights of the Andes, 3000 metresabove the level of the sea; and also, when on a low level,more considerable in the middle of continents than in theopen sea. This estimation is founded on the compara-tive noise of two volcanos, that of Guacamayo, and ofCotopaxi, which I have frequently heard both by dayand by night, one in a level plain between the city ofQuito and the savannahs of Chillo, and the other on thePacific Ocean, ten leagues west of the coast of Peru. The roarings of the volcanos of the Cordilleras usuallysucceed each other with great uniformity every five mi-nutes: they are not attended with any visible explosion |376| above the edges of the craters; and resemble, sometimesdistant thunder, at other times, repeated discharges ofheavy cannon. In places in which the ground is coveredin winter with snow in the vicinity of cascades, it wouldbe interesting to examine whether the nocturnal increaseof sound is not less in winter than in summer, when thesoil is strongly heated by the rays of the sun. It is to beobserved, that in what I have mentioned of the differ-ence between the daily and nightly sound from sonorousobjects in the high and low levels of South America, Ionly consider the comparative intensities under the samebarometric pressure. I do not compare the absolute in-tensity at different altitudes, but the difference betweenthe diurnal and nocturnal intensity on high levels and onplains. The variation of absolute intensity observed atdifferent heights of the atmosphere, is a problem whichthe mathematical theory of sound has long resolved. M.Poisson has even arrived at this remarkable result, thatthe intensity of sound from below upwards or from abovedownwards, whether the sonorous rays are propagatedvertically or obliquely, depends only on the density of thestratum of air whence the sound has proceeded. Onemust not therefore confound totally distinct problems. When the noise of the great cataracts of the Orinocois heard, at more than a league distance, in the plainthat surrounds the mission of Alures, the observer thinkshe is close to a coast surrounded with reefs and breakers.The noise is three times as great by night as by day, andgives an inexpressible charm to those solitary regions.What can be the cause of this nocturnal increase ofsound in a desert, in which nothing seems to break inupon the silence of nature? The diminution of tempera-ture, so far from increasing, would retard the velocityof sound. The intensity of sound diminishes in a wind |377| blowing in an opposite direction: it also diminishes by theexpansion of the air; it is weaker in the higher than inthe lower regions of the atmosphere, where the particlesof the agitated air have more density and more elasticityin the same ray. The intensity is the same in a dry airas in one full of vapour, but it is weaker in carbonic acidthan in the mixtures of azote and oxygen. On consider-ing these facts (the only ones that we know with cer-tainty), it is difficult to explain a phenomenon which isobserved near every cascade, and has been the subject ofcommon remark both in Europe and in America. Nearthe Orinoco the temperature of the air is lower by aboutfive degrees (Fahrenheit) by night than by day; whilst thevisible moisture increases at night, and the haze roundthe cataracts becomes denser. We have just seen thatthe hygroscopic state of the air has no influence on thepropagation of sound, whilst the velocity of the latter isdiminished by the cooling of the atmosphere. It might be supposed that, even in places remote fromthe habitation of man, the buzzing of insects, the sing-ing of birds, and the rustling of leaves in the gentlestwinds, produce, by day, a confused noise; which, beinguniform, may fill the ears without materially distractingthe attention, and thus imperceptibly lessen the effect ofany louder sound. But this reasoning, whatever be itsforce, can hardly apply to the forests of the Orinoco,when the air is constantly filled with an innumerablemultitude of moskitoes, so that the buzzing of insects ismuch stronger by night than by day; and where alsothe breeze, when it occurs at all, springs up after sun-set. I am inclined to think, that the presence of the sun af-fects the propagation and intensity of sound by the ob-stacles opposed to its transmission by currents of air of |378| different densities and partial undulations, the result ofthe unequal heating of various parts of the earth’s sur-face. In air at rest, whether it be dry, or mixed withelastic vapours equally distributed through it, the sonor-ous undulation is propagated without difficulty. Butwhen this air is crossed in every direction by small cur-rents of a warmer temperature, the sonorous undulationdivides into two waves, at the spot where there is themost sudden change in the density of the medium; thusproducing partial echoes, which weaken the body ofsound, because one of the sonorous waves is reflectedback upon itself. The theory of these partitions of sonor-ous waves has been lately explained by M. Poisson, withthe intelligence which distinguishes all his labours. *. Itis not, therefore, the motion of the passage of the parti-cles of air from below upwards, nor the small obliquecurrents of this fluid that we consider as opposing, byimpulse, the propagation of the sonorous waves. A stroke or impulse impressed on the surface of the liquid willform circles around the impinging centre, even when theliquid is in agitation. Several kinds of waves may crossin air, as well as in water, without interfering with eachother; but the true cause of the less intensity of soundin the day time appears to be the want of homogenietyin the elastic medium. There is at this time a suddenchange of density throughout, produced by small cur-rents of air, of a high temperature, rising from portionsof the earth’s surface that are unequally heated. Thesonorous waves are then divided in the same manner asluminous rays are refracted, and form a mirage of soundwherever strata of air of unequal density are contiguous.A distinction must be kept between the intensity of soundor of light, and the direction of the sonorous or luminous

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* Annales de Chimie, T. 7.|379| wave. When these waves are propelled across strata ofdifferent densities two simultaneous effects will be pro-duced, there will be a change in the direction of thewave, and extinction of light or sound. The reflexionthat accompanies each refraction weakens the intensityof light; the separation of the sonorous wave causes par-tial echoes, and that portion which returns on itself be-comes insensible to our ear, in weak noises, at the spotwhere the density of the medium suddenly changes. In the mirage with double images, that which has un-dergone refraction contiguous to the earth is alwaysweaker than the direct image. Strata of fluids, of verydifferent density, may so alternate, that the primitive di-rection of the luminous or the sonorous ray will remainthe same, but the intensity of the ray will be not the lessweakened on that account. During the night the surfaceof the earth cools; the parts covered with grass, or withsand take the same temperature; the atmosphere is nolonger crossed by currents of hot air, rising obliquely orvertically in every direction. The medium being nowbecome more homogeneous, the sonorous wave passeswith less difficulty, and the intensity of sound increases,as the separations of the sonorous waves and echoes be-come less frequent. To give a precise idea of the cause of these currents ofhot air, which rise in the day-time from a soil unequallyheated; I will mention some observations which I havemade under the Tropic. In the Llanos, or steppes of Ven-zuela, I have found the sand at two o’clock heated to126° Fahrenheit, and sometimes as high as 140°. Theair under the shade of a Bombax was 97°; in the sun,eighteen inches from the ground, 109°. At night thesand was only 82°, having lost at least 46°. Near thefalls of the Orinoco, the ground being covered with |380| grasses, its temperature rose in the day only to 86°, thatof the air being 79°, but at the same time the tempera-ture of beds of granite lying on the surface was 118°. Ihave published a great number of similar observations inmy account of experiments on the mirage at Cumana, atthe same period that Dr. Wollaston was employed on thesame subject in Europe. If I have given the true cause of the nightly increaseof sound, it will not be surprising that, under the torridzone, this increase is greater within land than on theopen sea; greater in the level plains than on the sides ofthe Cordilleras. The surface of the equatorial seas heatsuniformly, and not beyond 85° Fahrenheit, whilst that ofthe continents, being variously coloured, and composedof materials of very different powers of radiation, acquiresa heat of from 86 to 126°. Under the tropics the earthusually remains warmer during the night than the air: inthe temperate zones, on the other hand, in calm nights,the surface of the ground cools down to seven or eightdegrees below that of the air. In Europe, the tempera-ture, instead of diminishing as the distance from theground recedes, presents a regular progressive increaseto the height of fifty or sixty feet. It is not surprising,therefore, that the terrestrial refractions are sometimes inthe temperate zone almost as great by night as by day.There will always be strata of air, of different densities,resting horizontally one upon the other; but the streamsof warm air that cross the atmosphere obliquely, will bemore rare at night than in the day. At 3000 metres inheight, in that part of the Andes which is situated be-neath the equator, the mean temperature of the air is only57°, and the force of radiation towards a cloudless sky,through a dry and very pure air, hinders the soil fromheating considerably in the day time. |381| I shall not dwell farther on these local circumstances,it is sufficient to have made the general deduction of thenocturnal increase of sound from the theory of sonorousundulations and their divisions. The whole depends onthe want of homogeniety in the vertical columns of theatmosphere, which (according to the ingenious applica-tion that M. Arrago has made of the interference andneutralisation of the rays) is also the true cause of thescintillation of the stars. Besides, it is well known thatthe propagation of sound is sensibly altered when a stra-tum of hydrogen gas is made to rest above one of com-mon air, in a tube closed at one extremity. The mountaineers of the Alps, as well as those of theAndes, regard an unusual increase of sound in calmnights as a sure prognostic of a change in the weather.“It is about to rain,” say they, “for the noise of thetorrent comes nearer.” M. Deluc has endeavoured toexplain this phenomenon by a change in barometricpressure, and by an increased number of bubbles of airbreaking on the surface of the water. This explanationis forced, and but little satisfactory. I will not attemptanother hypothesis, but I will suggest the analogy thatexists between the prognostic derived from an increaseof sound, and from that of light. The inhabitants of themountains also expect a change of weather, when, in calmweather, the mountains covered with snow appear sud-denly to approach nearer to the observer, and when theiroutlines are unusually well defined upon the blue sky thatsurrounds them. Whatever be the constitution of theatmosphere that causes these appearances, it is not a lit-tle curious to observe in it a new analogy between themotions of the sonorous and the luminous undulations.