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 Neimes - - - - 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 a la Trinite 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 a in the Phoenix - 65 a in the Centaur - 96 a in the Peacock - 78 Acharnar - - 94 a in the Crane - 81 a in the Indian - 50 b - - - 75 b - - - 47 g - - - 58 a in the Toucan - 70