4 
M. Humboldt on Isothermal Lines^ and the 
termined by barometrical levelling, we are still ignorant of the 
height of Erzeroum, Bagdad, Aleppo, Teheran, Ispahan, 
Delhi and Lassa, above the level of the neighbouring seas. 
Notwithstanding the intimate relation in which we have lately 
stood with Persia and Candahar, this branch of knowledge has 
not made any progress in the last fifty years. 
W e are not authorised, however, on account of the decrease 
of temperature in the upper regions of the atmosphere, to con- 
found the mean temperatures of places which are not placed on 
the same level. In the Old World, good observations, which can 
alone be used for establishing empirical laws, are confined to an 
extent between the parallels of 30 and 70 degrees of latitude, 
and the meridians of 30° east longitude, and 20° of west longi- 
tude. The extreme points of this region are the island of Ma- 
deira, Cairo, and the North Cape. It is a zone which is only a 
thousand nautical leagues, (l|7th of the circumference of the 
globe,) from east to west, and which, containing the Basin of the 
Mediterranean, is the centre of the primitive civilisation of 
Europe. The extraordinary shape of this part of the world, 
the interior seas and other circumstances, so necessary for de- 
veloping the germ of cultivation among nations, have given to 
Europe a particular climate, very different from that of other 
regions placed under the same latitude. But as the physical 
sciences almost always bear the impress of the places where they 
began to be cultivated, we are accustomed to consider the distri- 
bution of heat observed in such a region, as the type of the laws 
which govern the whole globe. It is thus that, in geology, we 
have for a long time attempted to refer all volcanic phenomena 
to those of the volcanoes in Italy. In place of estimating me- 
thodically the distribution of heat, such as it exists on the sur- 
face of continents and seas, it has been usual to consider as 
real exceptions every thing which differs from the adopted type, 
or, by pursuing a method still more dangerous in investigating 
the laws of nature, to take the mean temperatures for every five 
degrees of latitude, confounding together places under different 
meridians. As this last method appears to exclude the influ- 
ence of extraneous causes, I shall first discuss it briefly before I 
proceed to point out the method, essentially different, which I 
have followed in my researches. 
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