Digitale Ausgabe

In an interesting memoir on the temperature of the differentparts of the torrid zone at the level of the sea, just publishedby Baron Humboldt in the Annales de Chimie, &c. for Sep-tember last, he has entered into an examination of the equa-torial temperature, in reply to the observations of Mr Atkin-son, to which we have already referred in a former article.The importance of this part of his paper is such as to meritthe particular attention of our meteorological readers. “The question,” says he, “of the equatorial temperature hasbeen recently discussed in a memoir published by Mr Atkinson,in the second volume of the Memoirs of the Astronomical Socie-ty of London (p. 137-183,) and which contains very judiciousconsiderations on several important points of meteorology. Thelearned author endeavours to deduce from my own observations,by employing the artifices of the most rigorous calculus, that themean temperature of the equator is not less than 84°.5 Fahr.and not 81°.5, as I have supposed in my essay on isothermallines. Kirwan made it 84°, and Dr Brewster in his ClimatericFormulæ has adopted 82°.8. * (Edinburgh Journal of Sci-ence, 1825, No. vii. p. 180.)

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* We have adopted 82°.8 as the equatorial temperature in the warmmeridian passing through Africa; but have retained 81°.5, Humboldt’s|137| If the equatorial temperature under consideration were thatof the equatorial zone surrounding the whole globe, and bound-ed by the parallel of 3° north and 3° south, we must first ex-amine the temperature of the equatorial ocean, for there is onlyone-sixth of the circumference of the globe which in that zonebelongs to terra firma. But the mean temperature of the ocean between the limitswe have mentioned, varies in general between 80°.24, and 82°.4.I say in general, for we sometimes find between these limitsmaxima restricted to zones scarcely a degree wide, andwhose temperature rises in different longitudes from 83°.7, to84°.7. I have observed this last temperature, which may beregarded as very high in the Pacific ocean, to the east of theGalapagos Islands, and recently M. Baron Dirckinck of Holm-feldt, a well-informed officer of the Danish navy, who, at myrequest, made a great number of thermometrical observations,has found (in lat. 2°.5′ N., long. 81.°54′ W.) almost in theparallel of Punta Guascama, the surface of the water at 87°.1.These maxima do not belong to the equator itself. They oc-cur sometimes to the north, and sometimes to the south of it,and often between the latitude of 2\( \frac{1}{2} \)°, and 6°. The greatcircle which passes through the points where the waters of thesea are the warmest, cuts the equator at an angle which seemsto vary with the sun’s declination. In the Atlantic ocean, wemay sometimes even pass from the northern to the southerntemperate zone, in the zone of the warmest waters, withoutobserving the thermometer rise above 82°.4. The maxima are

According to Perrins ‒ ‒ ‒	82° 76′
Churrucca ‒ ‒ ‒	83 66
Quevedo ‒ ‒ ‒	83 48
Rodman ‒ ‒ ‒	83 84
Dr Davy ‒ ‒ ‒	82 58
Mean,	83° 26′

The air which rests upon these equatorial waters is from1°.8 to 2°.7, colder than the ocean. It results from these facts,

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measure for the temperature of the equator in America and Asia. Thisdouble measure is a necessary result of the isothermal lines being regulatedby two poles of maximum cold.—Ed.|138| that over 5-6ths of the circumference of the globe, the equato-rial aqueous zone, instead of presenting a mean temperature of84°.5, has probably not one of 83°.3. Mr Atkinson himselfadmits, p. 171, that the union of the aqueous and continentalparts tends to diminish the mean temperature of the equator.But in confining himself to the continental plains of southAmerica, this philosopher adopts for the equatorial zone from1° to 3° south, and upon different theoretical suppositions,84°.56, or 87°.8. He founds this conclusion on the fact, thatat Cumana, in lat. 10°.27, the mean temperature is 81°.68, andthat, by the law of the increase of heat from the pole to theequator, an increase which depends on the square of the cosineof the latitude,* the mean temperature ought to be at leastabove 84°.56. Mr Atkinson finds a confirmation of this re-sult, by reducing to the level of the equatorial seas severaltemperatures which I had observed on the declivity of theCordilleras, to a height of 500 toises; and in employing cor-rections, which he believes to be due to the latitude, and to theprogressive diminution of heat in a vertical plane, he does notdissemble how much a part of these corrections is rendereduncertain by the position of places in vast plains, or in narrowvallies.—Mem. Astr. Soc. Vol. ii. p. 149, 158, 171, 172, 182,and 183. In studying in all its generality the problem of the distri-bution of heat on the surface of the globe, and in freeing itof the accessory consideration of localities, (for example of theeffects of the configuration, the colour and the geographic re-lation of the soil; of those of the predominance of certainwinds, of the proximity of seas, of the frequency of cloudsand fogs, and of the nocturnal radiation towards a sky more orless serene,) we shall find that the mean temperature of a stationdepends on the different ways in which the influence of themeridian altitude of the sun manifests itself. This altitudedetermines at once the duration of the semidiurnal arcs, thelength and the transparency of the portion of the atmospherewhich the rays traverse before reaching the horizon; the

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* The law of increase approaches much more nearly to that of the simplecosines of the latitude.—Ed.|139| quantity of the absorbed or heating rays (a quantity whichaugments rapidly when the angle of incidence, reckoned fromthe level of the surface, increases;) and lastly, the number ofsolar rays which a given horizon embraces. The law ofMayer, with all the modifications which have been introducedinto it for thirty years, is an empirical law, which representsthe generality of the phenomena by approximation, and oftenin a satisfactory manner; but it cannot be employed againstthe testimony of direct observations. If the surface of theglobe, from the equator to the parallel of Cumana, was adesert like that of Sahara, or a savanna uniformly covered withgrasses like the Llanos of Calobozo or of Apure, there wouldundoubtedly be an increment of mean temperature from 10\( \frac{1}{2} \)°.of latitude to the equator, but it is very probable that thisincrease does not amount to 2\( \frac{1}{2} \)° of Fahrenheit. M. Arago,whose important and ingenious researches extend to all thebranches of meteorology, has found, from direct experiments,that from a perpendicular incidence to 20° of zenith distancethe quantity of reflected light is nearly the same. He hasfound also that the photrometrical effect of solar light variesextremely little at Paris in the month of August, from noonto three o’clock, in spite of the changes in the length of thepath described by the rays which traverse the atmosphere. If I have fixed the mean temperature of the equator inround numbers at 81\( \frac{1}{2} \)°., it was to attribute to the equatorialzone, properly so called, from 3° N. to 3° S., the mean tem-perature of Cumana, 81°.86. This city, surrounded witharid sands, situated under a sky always serene, and whosethin vapours almost never resolve themselves into rains, pos-sesses a more burning climate than all the places which sur-round it, and which are like it on the level of the sea. Inadvancing southward in America, and to the equator, by theOrinoco and Rio Negro, the heat diminishes, not on accountof the elevation of the soil, which from the Fort of St Carlosis very little, but on account of the forests, the frequency ofrains, and the transparency of the atmosphere. It is to beregretted that travellers, even the most laborious, should beso little in a state to advance the progress of meteorology, byadding to our knowledge of mean temperatures. They do|140| not remain a sufficient time in the countries whose climatethey desire to know, and they collect for the annual meansonly observations which others have made, and most frequent-ly at hours and with instruments which are far from givingcorrect results. Owing to the constancy of the atmosphericphenomena under the zone nearest to the equator, a shortspace of time is without doubt sufficient to give approximate-ly the mean temperature at different heights above the level ofthe sea. I have always pursued this class of researches; but the on-ly precise result which I have been able to obtain, and which isdeduced from observations made twice a-day, is that of Cu-mana. (Compare with respect to the degree of confidencewhich the mean temperatures merit, Relat. Hist. tom. i. p. 411,547, 631-637, 584; tom. ii. p. 73, 418, 463; tom. iii. p.314-320, 371-382.) The true numerical elements of climatology can only be fix-ed by skilful persons established for a great number of yearsin different parts of the earth; and, in this respect, the intel-lectual generation which is preparing itself in the free part ofequatorial America, from the coast to 2000 toises of altitudeon the back and on the declivity of the Cordilleras, betweenthe parallels of the Isle of Chiloe and San Francisco in NewCalifornia, will have the happiest influence on the physicalsciences. In comparing what has been known for forty years on themean temperature of the equatorial regions with what we nowknow, we must be astonished at the slow progress of positiveclimatology. I do not know, at the present day, more than onemean temperature observed with any appearance of precision,between 3° north and 3° south lat., and it is that of St Louisde Maranham in Brazil, 2°.29′ S. lat., which Colonel AntonioPereira found from observations made in 1821, three timesa-day, (at 8h a. m., 4h p. m., and 11h p. m.) to be 81°.32.(Annaes das Sciences das Artes e das Letras, 1822, tom. xvi.Plate II. p. 55-80.) This is still 0° 54 less than the meantemperature of Cumana.* Below 10\( \frac{1}{2} \)° of lat. we know onlythe mean temperature of

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* See a preceding notice on this subject in p. 117 of this Number.|141|

	Lat.	Fahr.
Batavia, ‒ ‒	6° 12′ S.	80° 42
Cumana, ‒ ‒	10 27	81 86

Between 10\( \frac{1}{2} \)° of lat. and the extremity of the torrid zone,we have

	Lat.	Fahr.
Pondicherry, ‒ ‒	11° 55′ N	85°28
Madras, ‒ ‒	13 4	80 42
Manilla, ‒ ‒	14 36	78 08
Senegal, ‒ ‒	15 53	79 70
Bombay, ‒ ‒	18 56	80 06
Macao, ‒ ‒	22 12	73 94
Rio Janeiro, ‒ ‒	22 54 S.	74 30
The Havanna, ‒ ‒	23 9 N.	78 26

And after the observations of Pereira,

Maranham, ‒ ‒

2° 29′ S.

81° 32

It appears to result from these data, that the only place inthe equinoctial region whose mean temperature exceeds 81°86, is situated in 12° latitude. This is Pondicherry, whoseclimate can no more serve to characterise the equatorial re-gion than the Oasis of Mourzouk, where the unfortunateRitchie and Captain Lyon assure us that they saw, duringwhole months, (perhaps from the sand disseminated in theair,) the thermometer at 117° and 128°, can characterise theclimate of the temperate zone in the north of Africa. Thegreatest mass of tropical land is situated between 18° and 28°of north lat., and it is in that zone also, thanks to the esta-blishment of so many rich commercial towns, that we possessmost meteorological knowledge. The three or four degreesnearest to the equator are a terra incognita for climatology.We are still ignorant of the mean temperatures of GrandPara, Guayaquil, and even Cayenne. When we consider only the heat attained in a particularpart of the year, we find in the northern hemisphere the mostscorching climates under the tropic itself, and a little beyond it.At Abusheer, for example, in lat. 28\( \frac{1}{2} \)°, the mean temperatureof the month of July is 93° 2. In the Red Sea we find thethermometer at noon at 131°, and in the night at 109°. AtBenares, lat. 25° 20′, the heat reaches in summer 131°, whilstit descends in winter to 46° 96. These observations in India|142| were made with an excellent thermometer for maxima, bySix. The mean temperature of Benares is 77° 36. * The extreme heat which occurs in the southern portion ofthe temperate zone, between Egypt, Arabia, and the Gulf ofPersia, is the simultaneous effect of the configuration of thesurrounding lands, of the state of the surface, of the constanttransparency of the air deprived of aqueous vapours, and thelength of the days, which increase with the latitudes. Betweenthe tropics, even great heats are rare, and generally do notexceed at Bombay 91° and at Vera Cruz 95°. It is almostneedless to state, that in this note we have referred only to ob-servations made in the shade, and far from the reflection ofthe ground. At the equator, where the two solstitial heightsreach 66° 32′, the times of the sun’s passing the zenith aredistant from one another 186 days. At Cumana, the heightat the summer solstice is 76° 59, and that of the winter sol-stice 56° 5′, and the times of passing the zenith, 17th Apriland 26th August, are distant 131 days. Farther to the north,at the Havanna, we find the solstitial height in summer89° 41′, and of winter 43° 23′, and the distance of the passage(12th June and 1st July) 19 days. If these passages are notrecognized with the same evidence in the curve of the month,it is because their influence is marked in some places by theoccurrence of the rainy season, and other electrical phenomena.The sun is at Cumana during 109 days, or more exactly dur-ing 1275 hours, (from the 28th October till the 14th Febru-ary,) lower than under the equator, but in this interval itsmaximum of zenith distance does not exceed 33°.55. Theretardation in the sun’s progress, in approaching the tropics,increases the heat of places situated farther from the equator,particularly towards the confines of the torrid and temperatezones. Near the tropics, for example at the Havanna, lat.23° 9′, the sun employs twenty-four days to describe a degreeon each side of the zenith; under the equator it requires on-

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* Mr Prinsep makes the mean temperature of Benares in

• 182276°.81
• 182376 .40

and the greatest range from 111\( \frac{1}{2} \)° to 45°; the mean heat of a well thirty-sixfeet deep was 79°.71′.—Ed.|143| ly five days. Near the tropics, for example at the Havanna,(lat. 23°29,) the sun employs twenty-four days to describe adegree on each side of the zenith; under the equator it em-ploys only five days. At Paris, lat. 48° 50′, where the sundescends to the winter solstice, as far as 17° 42′; the solstitialheight in summer is 64°38′. The sun is consequently fromthe 1st of May till the 12th August, during the interval of103 days, or 1422 hours, as high at Paris as at Cumana at an-other epoch in the year. In comparing Paris to the Havanna,we find, in the first place, from the 26th March to the 17thSeptember, during 175 days, or 2407 hours, the sun as highas it is in any other season under the tropic of Cancer. Butin this interval of 175 days, the warmest month (July) has,from the register kept in the royal observatory from 1806 to1820, a mean temperature of 65°.48, whilst at Cumana, andat the Havanna, where the sun descends, in the first place, to56°5′, in the second, to 43°23′, the coldest month still gives,in spite of the long nights at Cumana, 79°.16, and at the Ha-vanna 70°.16 of mean heat. Under all zones, the temperatureof a part of the year is modified by the temperature of theseasons which precede it. Under the tropics the diminutionof the temperatures is very inconsiderable, because the earthhas received in the foregoing months a mass of mean heat,which is equivalent at Cumana to 80°.6, and at Havanna to25°.5. From the considerations which I have now explained, itdoes not appear to me probable that the equatorial tempera-ture ever reaches 84°.56, as is supposed by the learned andestimable author of the Memoir on Astronomical Refractions.Father Beza, who was the first traveller who recommendedobservations at the coldest and warmest hours of the day, be-lieved that he had found in 1686 and 1699, in comparingSiam, Malacca, and Batavia, “that the heat is not greaterunder the equator than under 14° of latitude.” I am of opi-nion that there is a difference, but that it is very small, andmasked by the effect of so many causes, which act simulta-neously on the mean temperature of a place. The observa-tions hitherto collected do not afford us any measure of a pro-|144| gressive increase between the equator and the latitude of Cu-mana.” Paris, September 1826.