Digitale Ausgabe

Caraccas, Dec. 14. 1799. A FEW weeks after my arrival in South America, Itranſmitted to Delambre an extract from my aſtronomicalobſervations, becauſe I hoped that ſome of them might beintereſting to the Board of Longitude: as I have, however,learned that the brig by which I ſent my letters was wreckednear Guadaloupe during the ſtorm which lately prevailed intheſe regions, I think it neceſſary to tranſmit to you a copyof them. After a paſſage of about ſix weeks, in the frigate Pizarro,I arrived at the coaſt of Paria. My plan was to proceed firſtto the Havannah, and thence to Mexiko; but I could notwithſtand the ſtrong deſire I had of ſeeing the wonders of theOronoko, and the lofty Cordilleras, which extend from thehigh land of Quito to the rivers of Guarapeche and Arco.As I have now returned from a very intereſting journey tothe interior of Paria, through the Cordilleras of Cocolar, Tu-meri, and Guiri, and to the ſettlement there of the Capuchins,never before viſited by any naturaliſt, during which I carriedwith me, on three mules, my aſtronomical inſtruments, viz.a quadrant by Bird, and ſextants, teleſcopes, and microme-ters, by Ramſden and Troughton, you will perhaps expectthat I have done a great deal for aſtronomy; but you knowthat this ſcience is too remote from my principal objects—philoſophy in general, geology, eudiometry, and the phyſi- |356| ology of plants and animals; and under the latitude of 10°it is impoſſible to labour ſo inceſſantly as under 49°. I choſerather to make a few obſervations with accuracy, than a greatmany imperfect ones; and to inſert them with great minute-neſs in my journals, that in caſe I ſhould die in the courſeof my travels, no doubt might remain in regard to theirexactneſs. In the two papers which I tranſmitted to Delambre, youwill find my obſervations made in Europe with Borda’s new inclinatorium. I found by it, that local circumſtances havea greater influence on the inclination of the magnetic needlethan on its declination; and that no connection ſeems toexiſt between the ſituation of the place of obſervation and theinclination. This I find confirmed in the new world in theinterior of New Andaluſia, and therefore the obſervationsmade by Nouet in Egypt ſeem to be eſtabliſhed. The de-clination, however, is affected by local cauſes, but much leſs:at ſea they are much more regular, and the variations aremuch more uniform. I have here given you my obſervations of the inclinationonly, which are true to 15′. During calms I obtained themat ſea with more accuracy, and the periods of the vibrationsof the needle could be determined very exactly. If the ſamenumber is counted in the ſame time during five or ſix repeatedtrials, and if this is ſtill the caſe when the inſtrument ismoved from its place, the reſults, in my opinion, may beconfidered as correct. Though calms are not uncommonbetween the tropics, during the courſe of forty days I wasable to make only ten very accurate obſervations.

Places of Observation, 1799.	Latitude.	Longitudefrom thefirst Meridian.	Inclination.		Magnetic Power.Number of Vibrationsin Ten Minutes.
			CentigradeDegrees.	OldDegrees.
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 ‒ ‒ ‒ ‒	43 29 0	9 24·5	76·15	68 32	237
In the AtlanticOcean 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

|357| In the year 1776, according to Cavallo, the inclination ofthe needle in the undermentioned latitudes and longitudeswas 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 themſelveswith obſerving the declination, I know no place in the earthwhere the declination has been determined with certainty to10″, and not ten where it has been determined to 1′. Whatuncertainty ſtill prevails reſpecting the real declination of theneedle at Paris! The ten obſervations which I made at ſea,will ſerve hereafter to determine whether the inclinationſpeedily changes. The longitude and latitude of the placesof obſervation were always accurately determined at the timeby means of a Ramſden’s ſextant, divided from 15 to 15 ſe-conds; and a Berthoud’s chronometer. You will ſee bythem that the inclination decreaſes very rapidly from thelatitude of 37°, and that it increaſes leſs towards the eaſtthan to the weſt from the latitude of 37° to 48°. It appears to me, that in the higher chain of the calca-reous mountains of this province, not far from the equator,ſmall elevations above the level of the ſea derange the in-clination much more ſtrongly than the higher mountains inthe Pyrenees and Old Caſtile. As a proof, I ſhall heregive obſervations made at four places which lie pretty nearlyin an arch from north to ſouth (comprehending 24′.)

Places.	Height abovethe Sea, Paris Feet.	Inclination.		Vibrations inTen Minutes.
		CentigradeDegrees.	OldDegrees.
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 drawnup for the men of ſcience who accompanied La Perouſe, thatthe intenſity of the magnetic power is the ſame all over theearth; as he aſcribes the ſmall variation which he found init at Cadiz, Teneriff, and Breſt, to the imperfection of thecompaſs. He requeſted me to verify this circumſtance. You |358| here ſee that the magnetic power is ſo different, that between Paris and Cumana it decreaſed from 245 to 229 vibrations inten minutes, though it does not decreaſe with the inclina-tion. This decreaſe cannot certainly be aſcribed to anychange in the goodneſs of the needle, or to other accidentalcauſes: for the ſame needle made in the ſame time at Paris 245 vibrations; at Girona, 232; at Barcelona, 245; at Va-lentia, 235; and, after a journey of ſeveral months, gave atthe ſame places exactly the ſame number of vibrations asbefore my departure. Theſe are always the ſame, in theopen fields, in the houſe, or in a cavern, ſo that the mag-netic power in any place is always the ſame, and remains along 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 makeany good obſervations of the declination. Notwithſtandingall the trouble I took, I could not find an azimuth compaſson which I could depend within 40′. This is the true reaſonwhy I have made no mention to you of the declination atſea. 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 goodobſervation, made in the year 1775 aboard an Engliſh ſhipfrom Liverpool, places this vaniſhing point in latitude 29°north, long. 66° 40′ weſt. I obſerved, with great care, thedeclination at two places on the coaſt of America with acompaſs by Lenoir, in which the needle is ſuſpended by athread, according to the method of Prony and Von Zach.At noon, October 1799, it was at Cumana, and twentyleagues further eaſt, 4° 13′ 45″ eaſt: at Caripe, the chiefplace of the Capuchin miſſion among the Chaimas andCaribs, 3° 15′ eaſt. During the earthquake at Cumana, on the 4th of No-vember 1799, the inclination of the magnetic needle wasaltered, but not the declination. The former, before theſhock, was 44°·20 of the new diviſion; after the ſhock itremained at 43°·35. The number of the vibrations of thedipping needle, however, was the ſame as before, viz. 229 inten minutes. This, united to other obſervations, ſeems in myopinion to prove, that during the earthquake this ſmall partof the ball of the earth was changed, and not the needle;for, in diſtricts where no ſigns of an earthquake ever appearedin the primitive chain of accumulated maſſes of granite, theinclination was as great after as before. On account of the intereſt which you take in every thing |359| that relates to navigation, the following obſervations will notbe 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 diſco-vering ſhallows at ſea; and am able to confirm, in the fulleſtmanner, what they have ſaid. I was aſtoniſhed to ſee howthe water became evidently colder as its depth decreaſed, andhow the neighbourhood of ſhallows, and of the coaſts, couldthereby be announced. The worſt ſpirit of wine thermo-meter, if only ſenſible, will therefore, in the hands of themoſt ignorant mariner, be a very uſeful inſtrument in thenight-time, and during ſtorms, or when it is difficult orimpoſſible to heave the lead. This obſervation I cannot tooſtrongly recommend to the attention of the Board of Longi-tude. Our whole crew were aſtoniſhed to ſee how ſpeedilythe thermometer fell, when we approached the large bankwhich extends from Tobago to Grenada, and eaſt from Mar-garita. Theſe obſervations may be made with more eaſe, asthe water of the ſea, in an extent of 12000 ſquare miles, hasalways the ſame temperature day and night; ſo that the moſtſenſible thermometer, during from four to ſix days ſail, doesnot riſe or fall above 0·3°. In the neighbourhood of ſhal-lows it was from 2 to 3 degrees, and more, colder. Thisobſervation of Franklin, hitherto forgotten, may at ſomeperiod be of great uſe to navigation; not that ſeamen ſhouldthrow aſide the lead entirely, and truſt to the thermometer,for this would be folly; but becauſe the obſervations may beſo eaſily repeated, and becauſe the thermometer will an-nounce the danger much ſooner than the lead, as the colderwater above the ſhallows leſſens the temperature in the waterin the neighbourhood. I can aſſert that this new mean isnot more uncertain than the log, and the helps already em-ployed in navigation. If the thermometer does not fall, na-vigators ought not to depend on that they are entirely ſecurefrom 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 in-correct manner; and to immerſe a thermometer in a bucketfilled with ſea water is certainly much eaſier. I have alſomeaſured ſeveral timeſ the ſpecific gravity and temperature ofthe ſea water, at the ſurface and at certain depths, by meansof Dollond’s balance and thermometers, which are placed incaſes furniſhed with a valve. As my inſtruments were com-pared with the beſt Pariſian ones, and as I could be more

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* Vol. iii. p. 32.|360| certain in regard to my longitudes than is uſual, the ſmallchart in which I intend to make known the reſult of theſeexperiments will be intereſting. In the latitude of 17° and18° north, there is a zone in the ſea between Africa and theWeſt Indies, where, without an extraordinary current pre-vailing, the water is denſer than under a leſs latitude. Thefollowing are ſome of my reſults reſpecting the temperatureof the ſea water:

North Lat.	Long. fromthe FirſtMeridian.	Temperature
		of the Sea Waterat 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
30 35	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 aproper opportunity I was obliged to bring it with me to thislarge capital of the province of Caraccas, which is ſituatedin a fertile valley abounding with cacao, cotton, and coffee,2400 feet above the level of the ſea, and in a climate per-fectly ſimilar to that of Europe. The thermometer in thenight-time falls to 11°, and in the day never riſes above 17°or 18°. The cloudy weather, which here daily follows the ſun’spaſſage over the meridian, makes the obſervation of corre-ſponding altitudes very difficult and uncertain, as the after-noon obſervation is often loſt. The cloudy weather after theearthquake of Cumana deprived me of an opportunity of ob-ſerving the immerſions of the ſecond ſatellite of Jupiter onthe 2d and 9th of November. I ſhall here give you the prin- |361| cipal determinations of the longitude, which I made by meansof 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 ‒	46 12
Bocca de Drago (not certain) ‒	44 23
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, northe earthquake, have had the leaſt influence on the regulardaily variation in the ſtate of the barometer; and that, ac-cording to C. Richard, this daily variation amounts at Su-rinam to two lines. Theſe obſervations are the more intereſting, as all the chartsof this part of the world are very incorrect; and the agree-ment of my longitude of Teneriff and Tobago, with the ob-ſervations of Borda and Chabert, within from 2″ to 5″ oftime, is a proof of the excellence of my chronometers. The ſerenity of the nights between the tropics gave me anopportunity of comparing with each other the ſtrength of thelight of the auſtral ſtars; which in ſome of them, ſuch asthe Crane, Altar, Toucan, and the Foot of the Centaur, ſeemsto have changed ſince the time of La Caille. I employedthe method propoſed by Dr. Herſchel, and diaphragms ofthe 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