Extract of a Letter from M.

Von Humboldt to Lalande.

Caraccas, Dec. 14. 1799.

A FEW weeks after my arrival in South America, I tranſmitted to Delambre an extract from my aſtronomical obſervations, becauſe I hoped that ſome of them might be intereſting to the Board of Longitude: as I have, however, learned that the brig by which I ſent my letters was wrecked near Guadaloupe during the ſtorm which lately prevailed in theſe regions, I think it neceſſary to tranſmit to you a copy of them.

After a paſſage of about ſix weeks, in the frigate Pizarro, I arrived at the coaſt of Paria.

My plan was to proceed firſt to the Havannah, and thence to Mexiko; but I could not withſtand the ſtrong deſire I had of ſeeing the wonders of the Oronoko, and the lofty Cordilleras, which extend from the high land of Quito to the rivers of Guarapeche and Arco.

As I have now returned from a very intereſting journey to the interior of Paria, through the Cordilleras of Cocolar, Tumeri, and Guiri, and to the ſettlement there of the Capuchins, never before viſited by any naturaliſt, during which I carried with me, on three mules, my aſtronomical inſtruments, viz. a quadrant by Bird, and ſextants, teleſcopes, and micrometers, by Ramſden and Troughton, you will perhaps expect that I have done a great deal for aſtronomy; but you know that this ſcience is too remote from my principal objectsphiloſophy in general, geology, eudiometry, and the phyſiology of plants and animals; and under the latitude of 10° it is impoſſible to labour ſo inceſſantly as under 49°. I choſe rather to make a few obſervations with accuracy, than a great many imperfect ones; and to inſert them with great minuteneſs in my journals, that in caſe I ſhould die in the courſe of my travels, no doubt might remain in regard to their exactneſs.

In the two papers which I tranſmitted to Delambre, you will find my obſervations made in Europe with Borda’s new inclinatorium.

I found by it, that local circumſtances have a greater influence on the inclination of the magnetic needle than on its declination; and that no connection ſeems to exiſt between the ſituation of the place of obſervation and the inclination.

This I find confirmed in the new world in the interior of New Andaluſia, and therefore the obſervations made by Nouet in Egypt ſeem to be eſtabliſhed.

The declination, however, is affected by local cauſes, but much leſs: at ſea they are much more regular, and the variations are much more uniform.

I have here given you my obſervations of the inclination only, which are true to 15′.

During calms I obtained them at ſea with more accuracy, and the periods of the vibrations of the needle could be determined very exactly.

If the ſame number is counted in the ſame time during five or ſix repeated trials, and if this is ſtill the caſe when the inſtrument is moved from its place, the reſults, in my opinion, may be confidered as correct.

Though calms are not uncommon between the tropics, during the courſe of forty days I was able to make only ten very accurate obſervations.

Places of Observation, 1799.

Latitude.

Longitude from the first Meridian.

Inclination.

Magnetic Power.

Number of Vibrations in Ten Minutes.

Centigrade Degrees.

Old Degrees.

Paris ‒ ‒ ‒ ‒ 48° 50′15″ 20° 0′ E 77·15° 69° 28′ 245 Nîmes ‒ ‒ ‒ ‒ 43 50 12 21 59 72·65 65 23 240 Montpellier ‒ ‒ 43 36 29 21 32·5 73·20 65 53 245 Marſeilles ‒ ‒ ‒ 42 17 49 23 3·5 72·40 63 10 240 Perpignan ‒ ‒ ‒ 42 41 53 20 33·5 72·55 65 18 248 Barcelona ‒ ‒ ‒ 41 23 8 19 52 71·80 64 37 245 Madrid ‒ ‒ ‒ 40 25 18 13 58 75·20 67 41 240 Valencia ‒ ‒ ‒ 39 28 55 17 29 70·70 63 38 235 Medina del Campo 73·50 66 9 240 Guadarama ‒ ‒ 73·50 66 9 240 Ferrol ‒ ‒ ‒ ‒ 4329 0 9 24·5 76·15 68 32 237 In the Atlantic Ocean between Africa and America. 38 52 15 3 40 75·18 67 40 242 37 14 10 3 30 74·90 67 30 242 32 15 54 2 52·5 71·50 64 21 — 25 15 0 0 36 W 67 60 18 239 21 36 0 5 39 64·20 57 49 237 20 8 0 8 34 63 56 42 236 14 20 0 28 3 58·80 52 55 239 12 34 0 33 14 50·15 45 8 234 10 46 0 41 24 46·40 41 46 229 10 59 30 44 31·5 46·50 41 57 237

In the year 1776, according to Cavallo, the inclination of the needle in the undermentioned latitudes and longitudes was as follows:

Latitude.

Longitude.

Inclination.

24° 24′ 18° 11′ 59° 10 22 52 44 12 0 37 38 30 3

Since Coulomb and Caſſini no longer employ themſelves with obſerving the declination, I know no place in the earth where the declination has been determined with certainty to 10″, and not ten where it has been determined to 1′. What uncertainty ſtill prevails reſpecting the real declination of the needle at Paris!

The ten obſervations which I made at ſea, will ſerve hereafter to determine whether the inclination ſpeedily changes.

The longitude and latitude of the places of obſervation were always accurately determined at the time by means of a Ramſden’s ſextant, divided from 15 to 15 ſeconds; and a Berthoud’s chronometer.

You will ſee by them that the inclination decreaſes very rapidly from the latitude of 37°, and that it increaſes leſs towards the eaſt than to the weſt from the latitude of 37° to 48°.

It appears to me, that in the higher chain of the calcareous mountains of this province, not far from the equator, ſmall elevations above the level of the ſea derange the inclination much more ſtrongly than the higher mountains in the Pyrenees and Old Caſtile.

As a proof, I ſhall here give obſervations made at four places which lie pretty nearly in an arch from north to ſouth (comprehending 24′.)

Places.

Height above the Sea, Paris Feet.

Inclination.

Vibrations in Ten Minutes.

Centigrade Degrees.

Old Degrees.

Cumana 24 44·20° 39° 47′ 229 Zueteppe 1111·2 43·30 38 58 229 Impoſſibile 1470· 43·15 38 50 233 Cumanacoa 636 43·20 38 53 228 Cocollar 2352 42·60 38 20 229

Borda is of opinion, as appears by the memorandum drawn up for the men of ſcience who accompanied La Perouſe, that the intenſity of the magnetic power is the ſame all over the earth; as he aſcribes the ſmall variation which he found in it at Cadiz, Teneriff, and Breſt, to the imperfection of the compaſs.

He requeſted me to verify this circumſtance.

You here ſee that the magnetic power is ſo different, that between Paris and Cumana it decreaſed from 245 to 229 vibrations in ten minutes, though it does not decreaſe with the inclination.

This decreaſe cannot certainly be aſcribed to any change in the goodneſs of the needle, or to other accidental cauſes: for the ſame needle made in the ſame time at Paris 245 vibrations; at Girona, 232; at Barcelona, 245; at Valentia, 235; and, after a journey of ſeveral months, gave at the ſame places exactly the ſame number of vibrations as before my departure.

Theſe are always the ſame, in the open fields, in the houſe, or in a cavern, ſo that the magnetic power in any place is always the ſame, and remains a long time without change; and ſeems to be a general power, like that of gravity.

I had the mortification of not being able at ſea to make any good obſervations of the declination.

Notwithſtanding all the trouble I took, I could not find an azimuth compaſs on which I could depend within 40′.

This is the true reaſon why I have made no mention to you of the declination at ſea.

The point, however, where the inclination vaniſhes, certainly lies further to the weſt than marked in Lambert’s chart, in the Berlin aſtronomical almanac for 1779.

A good obſervation, made in the year 1775 aboard an Engliſh ſhip from Liverpool, places this vaniſhing point in latitude 29° north, long. 66° 40′ weſt. I obſerved, with great care, the declination at two places on the coaſt of America with a compaſs by Lenoir, in which the needle is ſuſpended by a thread, according to the method of Prony and Von Zach.

At noon, October 1799, it was at Cumana, and twenty leagues further eaſt, 4° 13′ 45″ eaſt: at Caripe, the chief place of the Capuchin miſſion among the Chaimas and Caribs, 3° 15′ eaſt.

During the earthquake at Cumana, on the 4th of November 1799, the inclination of the magnetic needle was altered, but not the declination.

The former, before the ſhock, was 44°·20 of the new diviſion; after the ſhock it remained at 43°·35. The number of the vibrations of the dipping needle, however, was the ſame as before, viz.

229 in ten minutes.

This, united to other obſervations, ſeems in my opinion to prove, that during the earthquake this ſmall part of the ball of the earth was changed, and not the needle; for, in diſtricts where no ſigns of an earthquake ever appeared in the primitive chain of accumulated maſſes of granite, the inclination was as great after as before.

On account of the intereſt which you take in every thing that relates to navigation, the following obſervations will not be unacceptable.

I have carefully proved what Dr. Franklin and Captain Jonathan Williams aſſert in the Tranſactions of the American Society reſpecting the uſe of the thermometer for diſcovering ſhallows at ſea; and am able to confirm, in the fulleſt manner, what they have ſaid.

I was aſtoniſhed to ſee how the water became evidently colder as its depth decreaſed, and how the neighbourhood of ſhallows, and of the coaſts, could thereby be announced.

The worſt ſpirit of wine thermometer, if only ſenſible, will therefore, in the hands of the moſt ignorant mariner, be a very uſeful inſtrument in the night-time, and during ſtorms, or when it is difficult or impoſſible to heave the lead.

This obſervation I cannot too ſtrongly recommend to the attention of the Board of Longitude.

Our whole crew were aſtoniſhed to ſee how ſpeedily the thermometer fell, when we approached the large bank which extends from Tobago to Grenada, and eaſt from Margarita.

Theſe obſervations may be made with more eaſe, as the water of the ſea, in an extent of 12000 ſquare miles, has always the ſame temperature day and night; ſo that the moſt ſenſible thermometer, during from four to ſix days ſail, does not riſe or fall above 0·3°.

In the neighbourhood of ſhallows it was from 2 to 3 degrees, and more, colder.

This obſervation of Franklin, hitherto forgotten, may at ſome period be of great uſe to navigation; not that ſeamen ſhould throw aſide the lead entirely, and truſt to the thermometer, for this would be folly; but becauſe the obſervations may be ſo eaſily repeated, and becauſe the thermometer will announce the danger much ſooner than the lead, as the colder water above the ſhallows leſſens the temperature in the water in the neighbourhood.

I can aſſert that this new mean is not more uncertain than the log, and the helps already employed in navigation.

If the thermometer does not fall, navigators ought not to depend on that they are entirely ſecure from ſhallows; but if it falls, they muſt be on their guard.

This warning is certainly more valuable than our ſea charts, where the ſhallows are in general laid down in a very incorrect manner; and to immerſe a thermometer in a bucket filled with ſea water is certainly much eaſier. I have alſo meaſured ſeveral timeſ the ſpecific gravity and temperature of the ſea water, at the ſurface and at certain depths, by means of Dollond’s balance and thermometers, which are placed in caſes furniſhed with a valve.

As my inſtruments were compared with the beſt Pariſian ones, and as I could be more certain in regard to my longitudes than is uſual, the ſmall chart in which I intend to make known the reſult of theſe experiments will be intereſting.

In the latitude of 17° and 18° north, there is a zone in the ſea between Africa and the Weſt Indies, where, without an extraordinary current prevailing, the water is denſer than under a leſs latitude.

The following are ſome of my reſults reſpecting the temperature of the ſea water:

Vol. iii. p. 32.

North Lat. Long. from the Firſt Meridian.

Temperature of the Sea Water at the Surface. of the Air. 43° 29′ 9° 29′ E. 12° 18° 39 10 3 41·5 12 13 36 3 2 57 12 14 35 8 2 15 13 16·5 32 15 2 52·5 14·2 13·5 3035 3 6 15 16 28 55 2 37·5 15 17 26 51 — 47 16 15 20 8 8 33 W. 17 16 18 53 10 5 17·4 17 18 8 13 2 17·9 19 17 26 15 26 18 16 15 22 22 49 18·5 20 14 57 24 40 19 17 13 51 30 2·5 19·8 18·9 10 46 41 24 20·7 20·3 10 28 46 31 21 17 to 27 10 29 46 35 17·8 23

This letter was written at Cumana, but for want of a proper opportunity I was obliged to bring it with me to this large capital of the province of Caraccas, which is ſituated in a fertile valley abounding with cacao, cotton, and coffee, 2400 feet above the level of the ſea, and in a climate perfectly ſimilar to that of Europe.

The thermometer in the night-time falls to 11°, and in the day never riſes above 17° or 18°.

The cloudy weather, which here daily follows the ſun’s paſſage over the meridian, makes the obſervation of correſponding altitudes very difficult and uncertain, as the afternoon obſervation is often loſt.

The cloudy weather after the earthquake of Cumana deprived me of an opportunity of obſerving the immerſions of the ſecond ſatellite of Jupiter on the 2d and 9th of November. I ſhall here give you the principal determinations of the longitude, which I made by means of Berthoud’s chronometer, from the obſerved horary angles.

Long. weſt.

Cumana, caſtle of St. Antonio, lat. 10° 27′ 37″ and (taking the long.

of Madrid at 13° 58′) 46° 31′ Puerto Eſpana, in the iſland of Trinidad 43 49 30″ Tobago, the eaſtern extremity ‒ 42·47 30 Macanao, the weſt part of the iſland St. Margarita 46 35 30 Punto Araya, in New Andaluſia ‒ 46 35 30 Iſland of Coche, eaſtern extremity ‒ 4612 Bocca de Drago (not certain) ‒ 4423 Cabo de tres Puntas ‒ ‒ 44 54 30 Caraccas à la Trinité 10° 31′ 4″ latitude, very good.

I ſhall here alſo mention, that neither wind, ſtorms, nor the earthquake, have had the leaſt influence on the regular daily variation in the ſtate of the barometer; and that, according to C. Richard, this daily variation amounts at Surinam to two lines.

Theſe obſervations are the more intereſting, as all the charts of this part of the world are very incorrect; and the agreement of my longitude of Teneriff and Tobago, with the obſervations of Borda and Chabert, within from 2″ to 5″ of time, is a proof of the excellence of my chronometers.

The ſerenity of the nights between the tropics gave me an opportunity of comparing with each other the ſtrength of the light of the auſtral ſtars; which in ſome of them, ſuch as the Crane, Altar, Toucan, and the Foot of the Centaur, ſeems to have changed ſince the time of La Caille. I employed the method propoſed by Dr. Herſchel, and diaphragms of the ſame kind as thoſe uſed for the ſatellites.

If the light of Sirius be ſuppoſed equal to 100 parts, and that of Procyon 88, the light of the following ſtars, according to my obſervations, are as below expreſſed.

Parts.

Parts.

Canopus ‒ 98 α in the Phœnix ‒ 65 α in the Centaur ‒ 96 α in the Peacock ‒ 78 Acharnar ‒ ‒ 94 α in the Crane ‒ 81 α in the Indian ‒ 50 β ‒ ‒ ‒ 75 β ‒ ‒ ‒ 47 γ ‒ ‒ ‒ 58 α in the Toucan ‒ 70