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Art. V. — On Isothermal Lines , and the Distribution of Heat 
over the Globe. By Baron Alexander de Humboldt. 
(Concluded from Vol. IV. p. £81.) 
I SHALL now conclude this Memoir by the enumeration of 
the most important results which have been obtained by Baron 
Yon Each, M. Wahlenberg, and myself, on the distribution of 
heat in the interior of the earth, from the Equator to 70° of N. 
Lat., and from the plains to 8600 metres (11,808 feet) of ele- 
vation. I shall limit myself to an enumeration of the facts. 
The theory by which these facts are connected, will be found in 
the fine analytical work with which M. Fourier will soon enrich 
natural philosophy. 
The interior temperature of the earth is measured either by 
the temperature of subterraneous excavations, or by that of 
springs. This kind of observation is very liable to error, if the 
traveller does not pay the most minute attention to local circum- 
stances, which are capable of altering the results *. The air, 
when cooled, accumulates in caverns, which communicate with 
the atmosphere by perpendicular openings. The humidity of 
rocks depresses the temperature by the effect of evaporation. 
Caverns that have little depth are more or less warmed, accor- 
ding to the colour, the density, and the moisture, of the strata 
of stone in which nature has hollowed them. Springs indicate 
too low a temperature, if they descend rapidly from a consider- 
able height upon inclined strata. There are some under the 
torrid zone and in our climate, which do not vary in their tem- 
perature throughout the whole year more than half a degree ; 
and there are others which shew the mean temperature of the 
earth only by observing them every month, and taking the mean 
of all the observations. From the Polar circle to the Equator, 
and from the tops of mountains towards the plains, the progres- 
sive increase of the temperature of springs diminishes with the 
* Baron von Buch, in the Bibl. Brit. tom. xix. p. 263. ; Saussure, Voyages , 
sect. 1418. ; Wahlenberg, De Veget. Helvet. PI. 77— 84. ; Gilbert, Annalen , 1812, 
p. 150. 160. 277.; Lambert, Pyrometrie , p. 296. Dr Roebuck appears to have 
been the first who entertained exact notions on the temperature of springs, and 
upon their relation to the mean temperature of the air; Phil. Trans. 1775, vol. Ixv. 
p. 461. 
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