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| 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.