METEOROLOGY. 189 
Maine, 48°—50°, of Manheim, Vienna, Paris, Amsterdam, to 50° —52°, of 
Brussels, Hague, and Turin, 52°—531°, of Marseilles, Pekin, to 533°—55}°, 
of Mailand, Bordeaux, to 555°—57°, of Rome to 60°, and of Lisbon and 
Palermo to 61°—622°F. Pl. 29, fig. 2, contains a graphical representation 
of the mean annual temperatures in the cities of Berlin, Stockholm, and 
Copenhagen, from observations made during the years 1758—1834; jig. 3 
does the same for a period of ten years; fig. 4 is an exposition of the range 
of the hourly mean yearly temperature for two places in the north temperate 
zone, namely, Padua, and Fort Leith near Edinburgh. In the figures the 
degrees are given by Centigrade. 
The first clear view of the distribution of heat over the surface of the 
earth was given by Humboldt, by means of his tsothermal lines, lines 
connecting all those places whose mean annual temperature is the same. 
These are shown in pil. 29, fig. 1. as laid down on Mercator’s projection, 
with the thermometrical degrees by fives of Centigrade ; this projection also 
exhibits the proportion of rain. That isothermal obtained by connecting the 
hottest places of the earth, whose mean temperature is about 823°F"., is called 
the equator of heat. This is far from coinciding with the terrestrial equator, 
and lies almost entirely in the northern hemisphere. The curvature of the 
isothermals, which, within the tropics, are nearly parallel, becomes consi- 
derable in proportion as we remove from the equator. In the vicinity of 
the north pole, the isothermals probably form two separated and closed 
curves, whose centres may be taken as the coldest points in the northern 
hemisphere, and called poles of cold, after Brewster; neither of these coin- 
cides with the north pole, but one lies in Asia and the other in North America. 
Nevertheless it is not sufficient to know the mean annual temperature of 
a place to be acquainted with the distribution of heat in the different seasons, 
since places on the same isothermal line do not necessarily present the same 
climatic relations. Thus, Edinburgh and Tubingen have the same annual 
temperature of 473°F., yet the mean winter temperature of Edinburgh is 
38.4°F., and that of Tubingen 32.3°F. A good idea of the distribution of 
heat between summer and winter is obtained by constructing a map, in 
which all places of equal summer and all of equal winter temperature 
shall be connected by curves; the former are called isotheral, and the latter 
isocheimenal lines. Neither of these lines is parallel to the isothermals. 
The isocheimenal line of 32°F. passes through south-western Norway, 
Denmark, Bohemia, Hungary, Bessarabia, and the Crimea. The isotheral 
of 68°F. passes from the mouth of the Garonne, over Strasburg and Wurtz- 
burg, to Bohemia, through the Ukraine, north from the Caspian Sea, &c. 
The climate of a country, besides its geographical position, depends upon 
many circumstances, as the proximity of the sea and of mountain ranges ; 
the peculiarities of the soil, &c., exercise a great influence. We distinguish 
a land and a sea (island) climate: the land is warmed and cooled more 
readily and more quickly than the sea. For this reason the daily and 
annual variations of temperature are much more considerable in the interior 
of continents than over the sea, on the coast, and over islands and peninsulas, 
which have a more constant climate. Tobolsk and Irkutsk, in the interior 
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