| | | | It is not often that a life both full and long is granted to | man; but there are some bright exceptions to the general | rule, and amongst the most brilliant of these is Alexander | von Humboldt. Ohne hast, ohne Rast, | seems to have been his guiding motto; | | and to it he has adhered through every period of his life. | Of his unflagging, patient, and earnest industry we have an | example in the edition of his smaller works on which he is | at present engaged, and of which the first volume appeared | in the year 1853. It is dedicated to | Leopold von Buch, | | How touching are these words! | Few and simple though they be, what a novel picture do | they call up! Most fitting, too, is it thus to dedicate to his | friend an edition of works containing much that must have | frequently taken the venerable author's mind back to the | bygone days in which he and Leopold von Buch had often | taken pleasant counsel together respecting the subjects of | which these Essays treat. Ohne hast, | ohne Rast, they both pursued their path during those | fruitful sixty years, and it is but a little while ago that one | of them was called to rest from his labours. Loved and | revered be his memory by us, as by his still surviving | friend. | Baron Humboldt's object in publishing this collection of | his works has been to prevent others from undertaking a | compilation in which papers might appear which, in the | present state of science, would be of little or of only partial | value. His choice has principally fallen on such essays and | lectures as are scattered throughout periodical works, or | have been published in costly and not easily accessible | volumes; and in cases where fresh light has since been | thrown on the subjects treated of, he has mentioned the | fact in the addenda. Half of the present volume contains | papers on the Geography of the Cordilleras of South | America, and an account of two attempts to ascend | Chimborazo, which, together with the recital of an | expedition to the crater of the volcano of Pinchincha, we | recommend to the notice of all intending tourists who are | blases of Mont Blanc and | ennuyes of Vesuvius. There are also | three short treatises, one of Isothermal Lines, another on | the Constitution of our Atmosphere, and a third on the | Greater Intensity of Sound by Night, of which, as being a | very interesting paper in itself, and on a subject which | may be new to some of our readers, we propose to give an | abstract. | The phenomenon of the greater intensity of sound during | the night did not escape the notice of Aristotle, and it is | also alluded to by Plato in his | Dialogues. We are speaking, be it remembered, not | of that increase which is occasioned by a change of wind | and modified by the relation that subsists between the | direction of the wind and that of the waves of sound, but | the increase which takes place when the night is calm and | bright. Humboldt observed that, when he was travelling in | the tropics, it seemed to him to be greater in the plains | than on the summit of the Andes, at a height of 9000 or | 12,000 feet above the level of the sea ~~ greater also in the | interior of continents and in low-lying situations than at | sea. These estimates were founded upon his observations | of the sounds proceeding from two volcanoes, those of | Guacamayo and Cotopaxi, which he had an opportunity of | hearing by day and by night. | If anyone were to listen | to the roar of the great cataract of | the Orinoco, in the plains that surround the Mission | Atures, from a distance of more than four miles, he might | fancy himself to be near a heavy surge, breaking upon a | rocky shore. He would find, moreover, that the sound is | three times as loud during the night, and that it gives an | inexpressible and mysterious charm to those lonely and | desert solitudes, where there is nothing else to interrupt the | everlasting silence of Nature. It might, however, be | imagined that even in places uninhabited by man, the | buzzing of insects, the song of birds, the rustling of leaves | were no longer heard. But this is not the case in the woods | of the Orinoco, where the air is constantly filled with | clouds of mosquitoes, while the hum of insects is louder | by night than by day, and the wind only sets in after | sunset. The velocity of the propagation of sound is | lessened rather than increased by the lowering of the | temperature, and its intensity is diminished in an | atmosphere which has been set in motion by a wind the | direction of which is opposed to that of the waves of | sound. It is also diminished by the rarefaction of the air, | and is weaker in the higher regions of the atmosphere than | in the lower, in which the molecules of disturbed air are | more dense, and have less elasticity with the same radius. | The intensity is the same in moist and dry air, but it is | weaker in carbonic acid gas than in a mixture of oxygen | and nitrogen. | Having mentioned these facts, Baron Humboldt states that | he considers it probable that it is the presence of the sun | which influences the intensity of sound, by means of the | hindrance which the propagation of the waves meets with | from ascending currents of air of different densities, and | the partial oscillations of the atmosphere caused by the | unequal warming of different portions of the earth's | surface. During the night the ground is cooled, the | portions covered with sand or grass assume a uniform | temperature, and the atmosphere is no longer intersected | by little currents of warmer air which rise perpendicularly | or obliquely in all directions by day, dividing the waves of | sound and making them rebound upon themselves. In the | more uniform medium which is maintained during night, | the waves are propagated with less difficulty, and | therefore the intensity of sound increases ~~ the dividing | of the waves, and the partial echoes consequent upon their | rebound, less frequently taking place. If this cause be the | true one, it is not matter for surprise that in the tropical | zone the increase in the intensity of sound by night is | greater in the interior of the land than at sea, and greater in | the plains than on the mountains of the Cordilleras. The | surface of the equatorial seas is uniform, and rarely rises | above 84 degrees of temperature, whereas the surface of the | continent, varying in its conformation, and consisting of | substances that reflect heat in different degrees, has a | temperature varying from 86 degrees to 126 degrees. In the tropics, | also, the earth is generally warmer during the night than the air. | In the temperate zones, on the contrary, the ground, on | clear and calm nights, is often 7 degrees or | 9 degrees colder than the air; | and the temperature, instead of lessening in proportion to | the height from the ground, increases, in Europe, up to a | height of fifty or sixty feet. It is no wonder, therefore, that | the refraction of sound is sometimes almost as | considerable by night as by day in these regions. Here, | strata of air of different densities are constantly resting on | each other, but the small currents of warmer air which | ascend through the atmosphere in a oblique direction are | rarer by night than by day. At the height of 9000 feet, | those parts of the Andes lying under the equator have a | medium temperature of only 57 degrees, and the amount of | radiation in a dry and cloudless night prevents the ground | from becoming very much heated during the day. There is | no occasion, however, to dwell longer on these local | circumstances ~~ it is enough to have deduced, in general | terms, the cause of the increase of sound during the night | from the theory of the waves not coming so much in | contact with currents of different densities as they do by | day. Moreover, the mountaineers of the Alps, like those of | the Andes, look upon an extraordinary increase in the | intensity of sound heard on a quiet night as a sure sign of | change of weather.

"It will rain,"

| they say, ,p> "for we hear the | torrent more clearly."

They also predict a change of | weather when distant snow-covered mountains suddenly | seem near at hand, with their outlines clear out against the | sky. Whatever, therefore, may be the state of the | atmosphere which causes these phenomena, we may, at | any rate, perceive in these instances an analogy between | the propagation of the waves of sound and that of light. | In the notes added in 1853 to this Essay, Baron Humboldt | observes that the weaker intensity of sound upon the sea | cannot, as upon the continent, be caused by local | differences in the warmth of the surface, which he has | found to preserve very much the dame temperature over a | space of several hundred miles in the temperate zones; and | he therefore believes that it is only to be ascribed to the | want of elasticity in the fluid surface. The influence of | woollen draperies in a concert-room, and the deadening in | the report of a cannon let off at the edge of a corn-field in | full ear, present striking analogies to the diminution in the | loudness of sounds occurring on the sea. Thus, also, when | the intensity of sound is lessened during a snow-storm, he | considers it to be owing to an interruption in the | propagation of the waves of sound, caused by their coming | in contact with the different densities which they meet | with in their passage through the snow-laden medium. If, | however, the weakening of the sound which takes place | after a fall is only perceived, according to Derham, so long | as the snow is not covered with a coating of glittering ice, | it is an instance of the resonance of the surface of the | ground. | In connexion with this subject, it may be mentioned that | Saussure and others who have made the ascent of Mont | Blanc observed that, very early in their journey up the | mountain, the sound of their voices seemed to become | weakened in an extraordinary degree; whilst Humboldt, on | the occasion of Bonpland, Montufar and himself visiting | the volcanoes of Popayan, Quito, Peru, and Mexico, did | not remark anything of the kind, although they often | ascended to heights towering far above the summit of | Mont Blanc. On the contrary, they heard each other speak | with as much ease as if they had been at the level of the | sea; but then, he remarks, | He therefore considers that Bravais may be | right in thinking that the quickness with the ascent of | Mont Blanc is made affects the organ of hearing, and that | it is not the intensity of sound which is weakened. | For the first really useful and trustworthy results, founded | on numerical data, as to the intensity of sound, we have to | thank Messrs. Martin and Bravais. As a note struck by a | tuning-fork would always preserve the same intensity in | an aerial medium of uniform density, the difference of the | distance at which we would give an accurate measure of | its intensity. In a plain near the village of St. Cheron | (Dep. de Seine et Oise) the note struck by the tuning-fork | (performing at 512 vibrations in a second) ceased to be | heard at a distance of 833 feet. The state of the atmosphere | at the time was calm, and scarcely disturbed by a gentle | south wind which, however, cut at right angles the line | that separated the listener from the instrument. The sky | was cloudy, the temperature (half-past one o'clock, P.M., | 22nd June, 1844). 75 degrees, the barometer 29.3. The experiment | was repeated at midnight, and although the stillness was | more broken then | | than it had been even during the day by the hum of insects, | the fall of small twigs, and the barking of dogs, M. Martin | was able to perceive the sound 410 feet further off. It was | only when the observers had retreated to a distance of | 1243 feet that they ceased to hear it | We heartily recommend these Kleine | Schriften to general notice. We may add that this | Essay is followed by one on the Medium heights of the | Continents of Europe, America, and Asia; and lastly, we | have a comparison of the different rates of temperature at | the places or stations marked on the tables at the end of the | volume. We need scarcely say that the whole book is full | of interesting and valuable matter, as everything indeed | must be which comes from such a source.