Digitale Ausgabe

Sir,

THE generous interest taken by Your Royal Highness inthe advancement of human knowledge, encourages me tohope for the favourable reception of the request which with re-spectful confidence, I now venture to address to you. I takethe liberty of soliciting your attention to the labours requisitefor the investigation, by precise means, almost constantly em-ployed, of the variations of terrestrial magnetism. By obtainingthe cooperation of a great number of zealous observers, pro-vided with instruments of similar construction, M. Arago,

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† We translate this letter from Schumacher’s Astronomische Nachrichten,No. 306, which has been kindly communicated to us for the purpose.|43| Mr. Kupffer, and myself have succeeded in the last eight yearsin extending these researches over a very considerable partof the northern hemisphere. Permanent magnetic stations be-ing now established from Paris to China, following towardsthe east the parallels from 40° to 60°, I feel myself justifiedin soliciting, through the intervention of Your Royal Highness,the powerful cooperation of the Royal Society of London, tosanction this enterprise, and also to promote its success by theestablishment of new stations, as well in the vicinity of themagnetic equator as in the temperate part of the southernhemisphere. An object which is equally important whether it be consi-dered in connexion with the physics of the earth or the im-provement of nautical science, has a double claim upon theattention of a Society, which has from its commencement, withconstantly increasing success, cultivated the vast field of theexact sciences. Our information respecting the progressivedevelopment of the knowledge which we possess of terrestrialmagnetism must be indeed imperfect, if we are ignorant of thenumerous valuable observations which have been made at dif-ferent epochs, and are still being made, in the British isles,and in various parts of the equinoctial zone subject to thesame empire. Our present object is to render these observa-tions more useful, that is, better adapted to manifest greatphysical laws, by coordinating them according to a uniformplan, and connecting them with the observations now in pro-gress upon the continent of Europe and Northern Asia. Having been much occupied during my travels in the equi-noctial regions of America, during the years 1799—1804, withthe phænomena of the intensity of the magnetic forces, and theinclination and declination of the magnetic needle, on my re-turn to my own country I conceived the design of examiningthe progress of the horary variations of the declination, andthe perturbations to which it is liable, by employing a methodwhich, I believe, has never yet been followed upon an exten-sive scale. In a large garden at Berlin, during the years1806 and 1807, particularly at the period of the equinoxesand solstices, I measured the angular alterations of the mag-netic meridian, at intervals of an hour, often of half an hour,without interruption during four, five, and six days, and asmany nights. Mr. Oltmanns, whose numerous calculations ofgeographical positions have recommended him to the noticeof astronomers, kindly shared with me the fatigues of theselabours. The instrument which we employed was a magnetictelescope (lunette aimantée) of Prony, capable of being reversedupon its axis, suspended according to the method of Coulomb,|44| placed in a glass frame, and directed towards a very distantmeridian mark, the divisions of which, illuminated during thenight, indicated even six or seven seconds of horary variation.In verifying the habitual regularity of a nocturnal period,I was struck with the frequency of the perturbations, espe-cially of oscillations the amplitude of which extended beyondall the divisions of the scale, and which occurred repeatedlyat the same hours before sunrise, and the violent and ac-celerated movements of which could not be attributed to anyaccidental mechanical cause. These vagaries of the needle, thealmost periodical return of which has recently been confirmedby Mr. Kupffer in the narration of his Travels in the Caucasus,appeared to me the effect of a reaction of the interior of theearth towards the surface; I should venture to say, of magneticstorms, which indicate a rapid change of tension. From thattime it has been my desire to establish on the east and westof the meridian of Berlin apparatus similar to my own, inorder to obtain corresponding observations made at great di-stances and at the same hours; but the political tempest ofGermany, and my hasty departure for France, whither I wassent by the Government, delayed for a length of time the ex-ecution of this project. Fortunately my illustrious friendM. Arago, after his return from the coasts of Africa and theprisons of Spain, undertook, I think about the year 1818, aseries of observations upon magnetic declinations at the Ob-servatory of Paris, which, made daily at intervals uniformlyfixed, and continued upon the same plan to the present day,are considered, with regard to their number and mutual con-nexion, superior to everything that has been attempted inthis kind of physical investigations. Gambey’s apparatus,which is employed, is of perfect execution. Provided withmicrometers and microscopes, it may be employed with morecertainty and convenience than Prony’s instrument, which isattached to a strong magnetized bar of 20\( \frac{1}{4} \) inches in length. During the progress of these observations M. Arago hasdiscovered, and proved by numerous examples, a phænomenonwhich differs essentially from the observation made by Prof.Hiorter at Upsal in 1741. He has discovered not only thatthe Auroræ boreales disturb the regular progress of the horarydeclinations there when they are not visible, but also thatearly in the morning, often ten or twelve hours before the lu-minous phænomenon is developed in a very distant place, itsappearance is announced by the particular form presented bythe curve of the diurnal variations, that is, by the value of themaxima of elongation of the morning and night. Another newfact was manifested in the perturbations. Mr. Kupffer having|45| established at Cazan, nearly the eastern limit of Europe, oneof Gambey’s compasses, exactly similar to that employed byM. Arago at Paris, the two observers were convinced by acertain number of corresponding measures of horary declina-tion, that, notwithstanding a difference of longitude of morethan 47°, the perturbations were isochronous. They were likesignals which from the interior of the earth simultaneouslyarrived at its surface on the borders of the Seine and theWolga. When in 1827 I again fixed my residence at Berlin, myfirst care was to renew the series of observations which I hadmade at short intervals during the days and nights of theyears 1806 and 1807. I endeavoured at the same time togeneralize the means of simultaneous observations, the acci-dental employment of which had just produced results so im-portant. One of Gambey’s compasses was placed in the mag-netic pavilion, in which no portion of iron was introduced,which had been erected in the middle of a garden. Regularobservations could not commence till the autumn of 1828.Being called, in the spring of 1829, by His Majesty the Em-peror of Russia, to undertake a mineralogical tour in theNorth of Asia and on the Caspian Sea, I had an opportunityrapidly to extend the line of stations towards the east. Atmy request the Imperial Academy and the Curator of theUniversity of Cazan erected magnetic houses at St. Petersburghand Cazan. In a committee of the Imperial Academy, atwhich I had the honour of presiding, a discussion took placeon the immense advantages, with regard to our knowledge ofthe laws of terrestrial magnetism, presented by the vast extentof country limited on one side by the curve without declina-tion of Doskino, (between Moscow and Cazan, or with moreprecision, according to M. Adolphe Erman, between Osabli-kowo and Doskino, in lat. 56° 0′, and long. 40° 36′ eastof Paris,) and on the other, by the curve without declina-tion of Arsentchewa near Lake Baikal, which is believed tobe identical with that of Doskino, with a difference of meri-dians of 63° 21′. The Imperial department for Mines havinggenerously concurred in the same object, magnetic stationshave been successively established at Moscow, Barnaoul, theastronomical position of which I find to be at the foot ofAltai, in lat. 53° 19′ 21″, long. 5h 27′ 20″ east of Paris, andat Nertschinsk. The Academy of St. Petersburgh has donestill more, and has sent a courageous and clever astronomer,M. George Fuss, the brother of its perpetual secretary, toPekin, and has procured the erection there of a magneticpavilion, in the convent garden of the monks of the Greek|46| church. This undertaking cannot be mentioned without re-calling the fact, that, according to the Penthsaoyani, a me-dical natural history composed under the Soung dynasty,nearly four hundred years before Christopher Columbus andthe natives of Europe had the least idea of magnetic declina-tion, the Chinese suspended the needle by means of a thread,to allow it perfect freedom of motion; and that they knew thatwhen thus suspended, according to the method of Coulomb,(as in the Jesuit Lana’s apparatus in the seventeenth century,)the needle declined to the south-east, and never rested at thetrue south point. Since the return of M. Fuss, M. Kowanko,a young officer of mines, whom I had the pleasure to meetin the Oural, continues the observations of horary declination,corresponding to those of Germany, St. Petersburgh, Cazan,and Nicolajeff in the Crimea, where Admiral Greigh has esta-blished one of Gambey’s compasses, the care of which is con-fided to the director of the Observatory, Mr. Knorre. I havealso obtained the establishment of a magnetic apparatus at thedepth of thirty-five fathoms in an adit in the mines of Freibergin Saxony, where Mr. Reich to whom we are indebted for hisvaluable labours upon the mean temperature of the earth at dif-ferent depths, is assiduously engaged in making observations atregulated intervals. M. Boussingault, who neglects nothingwhich is calculated to advance the progress of the physics of theearth, has sent us from South America observations of horarydeclination made at Marmato, in the province of Antioquia, innorth lat. 5° 27′, in a place where the declination is eastern,as at Cazan and Barnaoul in Asia; while on the north-westerncoasts of the new continent, at Sitka in the Russian settle-ments, Baron von Wrangel, also provided with one of Gam-bey’s compasses, has taken part in the simultaneous observa-tions made at the time of the solstices and equinoxes. A Spanishadmiral, M. de Laborde, having been informed of a requestthat I had made to the Patriotic Society of the Havannah, hadthe kindness, unsolicited, to desire me to send him instrumentsproper for determining with precision the inclination, the ab-solute declination, and the horary variation of declination andintensity of the magnetic forces. The valuable instrumentsdesired, exactly similar to those in the possession of the Obser-vatory of Paris, arrived in safety in the island of Cuba; but thealteration in the maritime command at the Havannah, and otherlocal circumstances, have hitherto prevented the employmentof them, and the establishment of a magnetic station underthe tropic of Cancer. The same has also occurred up to thepresent time with regard to one of Gambey’s compasses whichM. Arago had caused to be erected, at his own expense, to|47| obtain observations in the interior of Mexico, where the soilis elevated six thousand feet above the level of the sea. Lastly,during my last residence in Paris, I had the honour of pro-posing to Admiral Duperré, Minister for marine affairs, theestablishment of a magnetic station in Iceland. The pro-posal was received with the utmost eagerness, and the instru-ment, which is already ordered, will be deposited during thepresent summer at the port of Reikiawig, when the expedi-tion which has been sent to the north in search of M. de Blosse-ville and his companions in misfortune returns to Iceland tocontinue its scientific labours. There cannot be any doubtthat the Danish Government, which protects with generousardour astronomy and the advancement of nautical science,will favour the establishment of a magnetic station in one ofits provinces bordering on the polar circle. At Chili alsoM. Gay has made a great number of corresponding horaryobservations, according to the instructions of M. Arago. I have entered upon this long and minute historical detail,to show how far I have hitherto succeeded, in conjunction withmy friends, in extending the number of simultaneous observa-tions. After my return from Siberia, Mr. Dove and I pub-lished, in 1830, a graphic delineation of the curves of horarydeclination of Berlin, Freiberg, Petersburgh, and Nicolajeffin the Crimea, to show the parallelism of these lines, notwith-standing the distance of the stations and the influence of ex-traordinary perturbations. In the comparison of the observa-tions of St. Petersburgh and Nicolajeff, use has been madeof observations taken at the very small intervals of twenty mi-nutes. It must not, however, be imagined that this parallelismof inflections always exists in the horary curves. We havefound that even in places very near to each other,—for instance,at Berlin and in the mines of Freiberg,—the magnetic reac-tions from the interior to the surface of the earth are not al-ways simultaneous; that one of the needles presents consider-able perturbations, while the other preserves that regularity,which under each meridian is the function of the true time ofthe place. In the memoir published in 1830, I proposed the fol-lowing periods for simultaneous observations at all the stations.

• • March 20th and 21st.
  • May 4th and 5th.
  • June 21st and 22nd.
  • Aug. 6th and 7th.
  • Sept. 23rd and 24th.
  • Nov. 5th and 6th.
  • Dec. 21st and 22nd.
  From four o’clock in the morning ofthe first day, to midnight of the se-cond day. The observations to becontinued at each magnetic stationduring the day and night, at inter-vals not exceeding one hour.

As several observers situated upon the line of the stations|48| have found these periods too near to each other, it has beenthought advisable to insist in preference upon the time of thesolstices and equinoxes. England, from the time of William Gilbert, Graham, andHalley to that of the more recent exertions of Messrs. Gilpin,Beaufoy (at Bushy), Barlow, and Christie, has produced arich collection of materials applicable to the discovery of thephysical laws which regulate the variation of the magnetic de-clination, either in one place according to the different hoursand seasons, or at various distances from the magnetic equatorand the lines without declination. Mr. Gilpin made observationsduring twelve hours every day for more than seven months.The numerous observations of Colonel Beaufoy were regularlypublished in Thomson’s Annals. The memorable expedi-tions to the most inhospitable regions of the North have fur-nished Messrs. Sabine, Franklin, Hood, Parry, Henry Foster,Beechey, and James Clarke Ross with a rich harvest of im-portant observations. Physical geography is indebted for aconsiderable increase of knowledge respecting terrestrial mag-netism and meteorology to the attempts which have recentlybeen made to determine the form of the north-west passageor strait; and to the perilous explorations of the frozen coastsof Asia by Captains Wrangel, Lütke, and Anjou. During theprogress of these noble efforts, an unexpected impulse hasbeen given to the physical sciences by the light thrown uponthem by a branch of natural philosophy the theoretical pro-gress of which for two centuries had been extremely slow.Such has been the effect of the grand discoveries of Oersted,Arago, Ampère, Seebeck, and Faraday upon the nature ofelectro-magnetic forces. Excited by the talents and ingeniousexertions of learned travellers cooperating for the promotionof one object, Messrs. Hansteen, Due, and Adolphus Erman,by the fortunate union of very precise astronomical and phy-sical means, have explored, throughout the immense extentof Northern Asia, the isoclinal, isogonal, and isodynamiccurves for very nearly the same epoch. When speaking ofthis great project, long since conceived and proposed by Mr.Hansteen, I ought, perhaps, to pass over in silence the obser-vations upon magnetic inclination which I made upon therarely-visited frontier of Chinese Dzoungarie and on thecoasts of the Caspian Sea, published in the second vo-lume of my Fragmens Asiatiques. My learned countrymanMr. Adolphus Erman, who embarked at Kamtschatka and re-turned to Europe by Cape Horn, had the rare advantage ofcontinuing throughout a long voyage the measure of the threemanifestations of terrestrial magnetism at the surface of the|49| globe. He employed the same instruments and the same me-thods which he had made use of from Berlin to the mouth ofthe Oby, and thence to the Sea of Okhotsk. That which characterizes our epoch, at a time distinguishedby grand discoveries in optics, electricity, and magnetism, isthe possibility of connecting phænomena by the generalizationof empirical laws, and the mutual aid afforded by scienceswhich had long remained isolated. At the present day simpleobservations upon horary declination or magnetic intensity,made simultaneously in situations very distant from each other,reveal, so to speak, what passes at profound depths in theinterior of our planet, and in the superior regions of the at-mosphere. The luminous emanations, the polar explosionswhich accompany the magnetic storm, appear to follow greatchanges in the habitual or mean tension of terrestrial mag-netism. It would tend greatly to promote the advancement of themathematical and physical sciences if, under the Presidencyand auspices of Your Royal Highness, the Royal Societyof London, to which I make it my boast to have belonged fortwenty years, would exert its powerful influence to extend theline of simultaneous observations, and to establish permanentmagnetic stations, either in the region of the tropics, on eachside of the magnetic equator, the proximity of which neces-sarily diminishes the amplitude of the horary declinations, orin the high latitudes of the southern hemisphere and in Ca-nada. I venture to propose this latter point, because obser-vations upon horary declination made in the vast extent of theUnited States are still very rare. Those, however, of Salem,in 1810, calculated by Mr. Bowditch, and compared by Aragowith the observations of Cassini, Gilpin, and Beaufoy, meritgreat praise, and might serve as a guide to observers in Ca-nada in investigating whether the declination there does notdiminish between the vernal equinox and the summer solstice,contrary to what occurs in Western Europe. In a memoirthat I published five years ago, I suggested as magnetic sta-tions extremely favourable to the progress of our knowledge,New Holland, Ceylon, the Mauritius, the Cape of GoodHope (rendered illustrious by the labours of Sir John Her-schel), St. Helena, and some point on the eastern coast ofAmerica to the south of Quebec. In the last century, in theyears 1794 and 1796, an English traveller, Mr. Macdonald,made some new and important observations upon the diurnalmotion of the needle at Sumatra and St. Helena, which havesince been confirmed and extended upon a large scale in thescientific expeditions of Captains Freycinet and Duperrey;|50| the former having the command of the sloop Uranie from1817 to 1820, and the latter, who has six times crossed themagnetic equator, commanding the sloop Coquille from 1822to 1825. To promote the rapid advancement of the theoryof terrestrial magnetism, or at least to establish with moreprecision empirical laws, it is necessary at the same time toprolong and to vary the lines of corresponding observations;also to distinguish in observations of horary variations, whatarises from the influence of the seasons, of serene and cloudyweather and of abundant rains, of the hours of day and night,and of the true time at each place, that is, from the influenceof the sun, and of all isochronous influences at the differentmeridians. To these observations of horary declination mustbe united those of the annual movement of the absolute declina-tion, of the inclination of the needle, and of the intensity ofthe magnetic forces, the increase of which from the magneticequator to the poles is unequal in the Western American andthe Eastern Asiatic hemispheres. All these data, indispensa-ble bases for the future theory, can only acquire certainty andimportance by the means of establishments which shall remainpermanent for a great number of years, of Physical Obser-vatories in which the investigation of numerical elements maybe repeated at settled intervals of time, and with similar in-struments. Travellers who cross a country in but one direc-tion and at one epoch merely prepare the way for an under-taking which should embrace the complete delineation of thelines without declination at intervals equally distant; the pro-gressive removal of the points of intersection of the terrestrialand magnetic equators; the changes of form in the isogonaland isodynamic lines; and the influence upon the slow or ac-celerated movement of the curves, which indubitably arisesfrom the configuration and articulation of the continents. Itmust be considered fortunate if the isolated labours of tra-vellers, whose cause it is my office to plead, have contributedto give animation to a species of investigation which is thework of centuries, and which requiries the concurrence ofnumerous observers, distributed according to a plan arrangedafter mature consideration, under the direction of several ofthe great scientific centres of Europe. The directors shouldnot always confine themselves to the narrow limits of thesame instructions, but they should vary them freely in adapta-tion to the progressive state of physical science, and the im-provement of instruments and methods of observation. When soliciting Your Royal Highness to condescend tocommunicate this letter to the illustrious Society over whichyou preside, it is not in any degree my office to inquire, which|51| are the magnetic stations that merit preference at the presenttime, or that local circumstances may admit of establishing.To have solicited the concurrence of the Royal Society ofLondon will be sufficient to give new life to a useful enter-prise in which I have been engaged for very many years. Iventure simply to express the wish that, should my proposi-tion be received with indulgence, the Royal Society wouldenter into direct communication with the Royal Society ofGöttingen, the Royal Institute of France, and the ImperialAcademy of Russia, in order to adopt measures the bestadapted for the combination of what it may be proposed toestablish with what already exists upon a very considerableextent of surface. Perhaps also measures might be previouslyconcerted for the publication of partial observations, and also(if the calculation would not require too much time, and toomuch retard the communications,) of the mean results. One ofthe happy effects of civilization and the progress of reason is,that when addressing learned societies, their willing concur-rence may be relied upon if the object for which it is solicitedtends to promote the advancement of the sciences or the intel-lectual development of humanity. Labours of astonishing precision have been performed,within the last few years, with instruments of extraordinarypower, in a magnetic pavilion of the Observatory of Göttin-gen, which are well worthy of the attention of philosophers,as they offer a more precise method of measuring the horaryvariations. The magnetized bar is of much larger dimensionsthan even the bar of Prony’s magnetic telescope; and the ex-tremity is furnished with a mirror, in which are reflected thedivisions of a scale which is more or less distant, according tothe angular value desired to be given to these divisions. Bythe employment of this improved method the necessity for theobserver’s approaching the magnetized bar is obviated, andby preventing the currents of air produced by the proximityof the human body, or during the night, of a lamp, observa-tions may be made in the smallest intervals of time. Thegreat geometrician Mr. Gauss,—to whom we owe this mode ofmaking observations, as well as the means of reducing the in-tensity of the magnetic force in any part of the earth to anabsolute proportion, and the ingenious invention of a magneto-meter put into motion by a multiplier of induction,—pub-lished in the years 1834 and 1835 several series of simulta-neous observations made with similar apparatus, and at inter-vals of five or ten minutes, at Göttingen, Copenhagen, Altona,Brunswick, Leipzig, Berlin (where Mr. Encke has alreadyestablished a very spacious magnetic house, near the NewRoyal Observatory), Milan, and Rome. Mr. Schumacher’s|52| German Ephemeris (Jahrbuch für 1836) proves graphically,and by the parallelism of the smallest inflections of the horarycurves, the simultaneity of the perturbations at Milan andCopenhagen, two cities having a difference of latitude of 10° 13′. Mr. Gauss first made observations at the times which I pro-posed in 1830, but with the intention of referring the angulardimensions of magnetic declination to the smallest intervals oftime. (On the 7th of February 1834, alterations of six minutesof the arc corresponded to a single minute of time.) Mr. Gaussreduced the forty-four hours of simultaneous observationsto twenty-four hours; and appointed six [seven?] periods ofthe year, viz. the last Saturday of each month consisting ofan uneven number of days, for the stations which are providedwith his new apparatus. The small magnetized bars whichhe employs as magnetometers are of four pounds weight, andthe large ones of twenty-five pounds. The curious apparatusof induction proper to render sensible and measurable theoscillatory movements predicted by a theory founded uponMr. Faraday’s admirable discovery, consists of two bars fast-ened together, each of twenty-five pounds weight. I thoughtit proper to mention the valuable labours of Mr. Gauss, inorder that those members of the Royal Society of Londonwho have rendered most service to the study of terrestrialmagnetism, and who know the localities of the colonial esta-blishments, may take into consideration whether bars of greatweight, provided with a mirror, and suspended in a pavilioncarefully closed, should be employed in the new stations to beestablished; or whether Gambey’s compass, hitherto uniformlyused in our present stations in Europe and Asia, should stillbe employed. In discussing this question the advantages willundoubtedly be estimated which, in the apparatus of Mr.Gauss, arise from the smaller mobility of the bars by currents ofair, as well as from the facility and rapidity with which theangular divisions may be read in very short intervals of time.My desire is only to see the line of magnetic stations extended,whatever be the means by which the precision of the corre-sponding observations may be attained. I ought also to men-tion that two accomplished travellers, Messrs. Sartorius andListing, provided with very portable instruments of small di-mensions, have very successfully employed the method of thegreat geometrician of Göttingen in their excursions to Na-ples and in Sicily.* Your Royal Highness will, I hope, excuse the length of this

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* An abstract of a memoir by Prof. Gauss in which his apparatus and me-thod of observation are fully described will be found in Lond. & Edinb. Phil.Mag., vol. ii. p. 291, et seq.—Edit.|53| communication; but I thought that it would be useful to uniteunder one point of view what has been done or proposed indifferent countries towards the attainment of extensive simul-taneous observations upon the laws of terrestrial magnetism.

Accept, Sir, the acknowledgement of the profound respectwith which I have the honour of being,Your Royal Highness’s, &c. &c., Berlin, April, 1836. Alexander von Humboldt.