Observations on the Mean Temperature of the Equatorial Regions. By Baron Alexander Humboldt. In an interesting memoir on the temperature of the different parts of the torrid zone at the level of the sea, just published by Baron Humboldt in the Annales de Chimie, &c. for September last, he has entered into an examination of the equatorial temperature, in reply to the observations of Mr Atkinson, to which we have already referred in a former article. The importance of this part of his paper is such as to merit the particular attention of our meteorological readers. “The question,” says he, “of the equatorial temperature has been recently discussed in a memoir published by Mr Atkinson, in the second volume of the Memoirs of the Astronomical Society of London (p. 137-183,) and which contains very judicious considerations on several important points of meteorology. The learned author endeavours to deduce from my own observations, by employing the artifices of the most rigorous calculus, that the mean temperature of the equator is not less than 84°.5 Fahr. and not 81°.5, as I have supposed in my essay on isothermal lines. Kirwan made it 84°, and Dr Brewster in his Climateric Formulæ has adopted 82°. 8. (Edinburgh Journal of Science, 1825, No. vii. p. 180.) We have adopted 82°.8 as the equatorial temperature in the warm meridian passing through Africa; but have retained 81°.5, Humboldt’s measure for the temperature of the equator in America and Asia. This double measure is a necessary result of the isothermal lines being regulated by two poles of maximum cold.—Ed. If the equatorial temperature under consideration were that of the equatorial zone surrounding the whole globe, and bounded by the parallel of 3° north and 3° south, we must first examine the temperature of the equatorial ocean, for there is only one-sixth of the circumference of the globe which in that zone belongs to terra firma. But the mean temperature of the ocean between the limits we have mentioned, varies in general between 80°.24, and 82°. 4. I say in general, for we sometimes find between these limits maxima restricted to zones scarcely a degree wide, and whose temperature rises in different longitudes from 83°.7, to 84°. 7. I have observed this last temperature, which may be regarded as very high in the Pacific ocean, to the east of the Galapagos Islands, and recently M. Baron Dirckinck of Holmfeldt, a well-informed officer of the Danish navy, who, at my request, made a great number of thermometrical observations, has found (in lat. 2°.5′ N., long. 81. °54′ W.) almost in the parallel of Punta Guascama, the surface of the water at 87°. 1. These maxima do not belong to the equator itself. They occur sometimes to the north, and sometimes to the south of it, and often between the latitude of 2 [Formel] °, and 6°. The great circle which passes through the points where the waters of the sea are the warmest, cuts the equator at an angle which seems to vary with the sun’s declination. In the Atlantic ocean, we may sometimes even pass from the northern to the southern temperate zone, in the zone of the warmest waters, without observing the thermometer rise above 82°. 4. The maxima are According to Perrins ‒ ‒ ‒ 82° 76′ Churrucca ‒ ‒ ‒ 8366 Quevedo ‒ ‒ ‒ 8348 Rodman ‒ ‒ ‒ 8384 Dr Davy ‒ ‒ ‒ 8258 Mean, 83° 26′ The air which rests upon these equatorial waters is from 1°.8 to 2°.7, colder than the ocean. It results from these facts, that over 5-6ths of the circumference of the globe, the equatorial aqueous zone, instead of presenting a mean temperature of 84°.5, has probably not one of 83°. 3. Mr Atkinson himself admits, p. 171, that the union of the aqueous and continental parts tends to diminish the mean temperature of the equator. But in confining himself to the continental plains of south America, this philosopher adopts for the equatorial zone from 1° to 3° south, and upon different theoretical suppositions, 84°.56, or 87°. 8. He founds this conclusion on the fact, that at Cumana, in lat. 10°.27, the mean temperature is 81°.68, and that, by the law of the increase of heat from the pole to the equator, an increase which depends on the square of the cosine of the latitude, the mean temperature ought to be at least above 84°.56. Mr Atkinson finds a confirmation of this result, by reducing to the level of the equatorial seas several temperatures which I had observed on the declivity of the Cordilleras, to a height of 500 toises; and in employing corrections, which he believes to be due to the latitude, and to the progressive diminution of heat in a vertical plane, he does not dissemble how much a part of these corrections is rendered uncertain by the position of places in vast plains, or in narrow vallies.—Mem. Astr. Soc. Vol. ii. p. 149, 158, 171, 172, 182, and 183. The law of increase approaches much more nearly to that of the simple cosines of the latitude.—Ed. In studying in all its generality the problem of the distribution of heat on the surface of the globe, and in freeing it of the accessory consideration of localities, (for example of the effects of the configuration, the colour and the geographic relation of the soil; of those of the predominance of certain winds, of the proximity of seas, of the frequency of clouds and fogs, and of the nocturnal radiation towards a sky more or less serene,) we shall find that the mean temperature of a station depends on the different ways in which the influence of the meridian altitude of the sun manifests itself. This altitude determines at once the duration of the semidiurnal arcs, the length and the transparency of the portion of the atmosphere which the rays traverse before reaching the horizon; the quantity of the absorbed or heating rays (a quantity which augments rapidly when the angle of incidence, reckoned from the level of the surface, increases;) and lastly, the number of solar rays which a given horizon embraces. The law of Mayer, with all the modifications which have been introduced into it for thirty years, is an empirical law, which represents the generality of the phenomena by approximation, and often in a satisfactory manner; but it cannot be employed against the testimony of direct observations. If the surface of the globe, from the equator to the parallel of Cumana, was a desert like that of Sahara, or a savanna uniformly covered with grasses like the Llanos of Calobozo or of Apure, there would undoubtedly be an increment of mean temperature from 10 [Formel] °. of latitude to the equator, but it is very probable that this increase does not amount to 2 [Formel] ° of Fahrenheit. M. Arago, whose important and ingenious researches extend to all the branches of meteorology, has found, from direct experiments, that from a perpendicular incidence to 20° of zenith distance the quantity of reflected light is nearly the same. He has found also that the photrometrical effect of solar light varies extremely little at Paris in the month of August, from noon to three o’clock, in spite of the changes in the length of the path described by the rays which traverse the atmosphere. If I have fixed the mean temperature of the equator in round numbers at 81 [Formel] °., it was to attribute to the equatorial zone, properly so called, from 3° N. to 3° S., the mean temperature of Cumana, 81°.86. This city, surrounded with arid sands, situated under a sky always serene, and whose thin vapours almost never resolve themselves into rains, possesses a more burning climate than all the places which surround it, and which are like it on the level of the sea. In advancing southward in America, and to the equator, by the Orinoco and Rio Negro, the heat diminishes, not on account of the elevation of the soil, which from the Fort of St Carlos is very little, but on account of the forests, the frequency of rains, and the transparency of the atmosphere. It is to be regretted that travellers, even the most laborious, should be so little in a state to advance the progress of meteorology, by adding to our knowledge of mean temperatures. They do not remain a sufficient time in the countries whose climate they desire to know, and they collect for the annual means only observations which others have made, and most frequently at hours and with instruments which are far from giving correct results. Owing to the constancy of the atmospheric phenomena under the zone nearest to the equator, a short space of time is without doubt sufficient to give approximately the mean temperature at different heights above the level of the sea. I have always pursued this class of researches; but the only precise result which I have been able to obtain, and which is deduced from observations made twice a-day, is that of Cumana. (Compare with respect to the degree of confidence which 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 fixed by skilful persons established for a great number of years in different parts of the earth; and, in this respect, the intellectual generation which is preparing itself in the free part of equatorial America, from the coast to 2000 toises of altitude on the back and on the declivity of the Cordilleras, between the parallels of the Isle of Chiloe and San Francisco in New California, will have the happiest influence on the physical sciences. In comparing what has been known for forty years on the mean temperature of the equatorial regions with what we now know, we must be astonished at the slow progress of positive climatology. I do not know, at the present day, more than one mean temperature observed with any appearance of precision, between 3° north and 3° south lat., and it is that of St Louis de Maranham in Brazil, 2°.29′ S. lat., which Colonel Antonio Pereira found from observations made in 1821, three times a-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 mean temperature of Cumana. Below 10 [Formel] ° of lat. we know only the mean temperature of See a preceding notice on this subject in p. 117 of this Number. Lat. Fahr. Batavia, ‒ ‒ 6° 12′ S. 80° 42 Cumana, ‒ ‒ 10 27 81 86 Between 10 [Formel] ° 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 in the equinoctial region whose mean temperature exceeds 81° 86, is situated in 12° latitude. This is Pondicherry, whose climate can no more serve to characterise the equatorial region than the Oasis of Mourzouk, where the unfortunate Ritchie and Captain Lyon assure us that they saw, during whole months, (perhaps from the sand disseminated in the air,) the thermometer at 117° and 128°, can characterise the climate of the temperate zone in the north of Africa. The greatest mass of tropical land is situated between 18° and 28° of north lat., and it is in that zone also, thanks to the establishment of so many rich commercial towns, that we possess most meteorological knowledge. The three or four degrees nearest to the equator are a terra incognita for climatology. We are still ignorant of the mean temperatures of Grand Para, Guayaquil, and even Cayenne. When we consider only the heat attained in a particular part of the year, we find in the northern hemisphere the most scorching climates under the tropic itself, and a little beyond it. At Abusheer, for example, in lat. 28 [Formel] °, the mean temperature of the month of July is 93° 2. In the Red Sea we find the thermometer at noon at 131°, and in the night at 109°. At Benares, lat. 25° 20′, the heat reaches in summer 131°, whilst it descends in winter to 46° 96. These observations in India were made with an excellent thermometer for maxima, by Six. The mean temperature of Benares is 77° 36. Mr Prinsep makes the mean temperature of Benares in and the greatest range from 111 [Formel] ° to 45°; the mean heat of a well thirty-six feet deep was 79°.71′.—Ed. 1822 76°.81 1823 76 .40 The extreme heat which occurs in the southern portion of the temperate zone, between Egypt, Arabia, and the Gulf of Persia, is the simultaneous effect of the configuration of the surrounding lands, of the state of the surface, of the constant transparency of the air deprived of aqueous vapours, and the length of the days, which increase with the latitudes. Between the tropics, even great heats are rare, and generally do not exceed at Bombay 91° and at Vera Cruz 95°. It is almost needless to state, that in this note we have referred only to observations made in the shade, and far from the reflection of the ground. At the equator, where the two solstitial heights reach 66° 32′, the times of the sun’s passing the zenith are distant from one another 186 days. At Cumana, the height at the summer solstice is 76° 59, and that of the winter solstice 56° 5′, and the times of passing the zenith, 17th April and 26th August, are distant 131 days. Farther to the north, at the Havanna, we find the solstitial height in summer 89° 41′, and of winter 43° 23′, and the distance of the passage (12th June and 1st July) 19 days. If these passages are not recognized with the same evidence in the curve of the month, it is because their influence is marked in some places by the occurrence of the rainy season, and other electrical phenomena. The sun is at Cumana during 109 days, or more exactly during 1275 hours, (from the 28th October till the 14th February,) lower than under the equator, but in this interval its maximum of zenith distance does not exceed 33°.55. The retardation 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 temperate zones. Near the tropics, for example at the Havanna, lat. 23° 9′, the sun employs twenty-four days to describe a degree on each side of the zenith; under the equator it requires only five days. Near the tropics, for example at the Havanna, (lat. 23°29,) the sun employs twenty-four days to describe a degree on each side of the zenith; under the equator it employs only five days. At Paris, lat. 48° 50′, where the sun descends to the winter solstice, as far as 17° 42′; the solstitial height in summer is 64°38′. The sun is consequently from the 1st of May till the 12th August, during the interval of 103 days, or 1422 hours, as high at Paris as at Cumana at another epoch in the year. In comparing Paris to the Havanna, we find, in the first place, from the 26th March to the 17th September, during 175 days, or 2407 hours, the sun as high as it is in any other season under the tropic of Cancer. But in this interval of 175 days, the warmest month (July) has, from the register kept in the royal observatory from 1806 to 1820, a mean temperature of 65°.48, whilst at Cumana, and at the Havanna, where the sun descends, in the first place, to 56°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 Havanna 70°.16 of mean heat. Under all zones, the temperature of a part of the year is modified by the temperature of the seasons which precede it. Under the tropics the diminution of the temperatures is very inconsiderable, because the earth has received in the foregoing months a mass of mean heat, which is equivalent at Cumana to 80°.6, and at Havanna to 25°.5. From the considerations which I have now explained, it does not appear to me probable that the equatorial temperature ever reaches 84°.56, as is supposed by the learned and estimable author of the Memoir on Astronomical Refractions. Father Beza, who was the first traveller who recommended observations at the coldest and warmest hours of the day, believed that he had found in 1686 and 1699, in comparing Siam, Malacca, and Batavia, “that the heat is not greater under the equator than under 14° of latitude.” I am of opinion that there is a difference, but that it is very small, and masked by the effect of so many causes, which act simultaneously on the mean temperature of a place. The observations hitherto collected do not afford us any measure of a progressive increase between the equator and the latitude of Cumana.” Paris, September 1826.