324 COSMOS. 



trical pressure, the calm state of the air or the action of oppo- 

 site winds, the amount of electric tension, the purity of the 

 atmosphere or its admixture with more or less noxious gase- 

 ous exhalations, and, finally, the degree of ordinary trans- 

 parency and clearness of the sky, which is not only important 

 with respect to the increased radiation from the earth, the 

 organic development of plants, and the ripening of fruits, but 

 also with reference to its influence on the feelings and mental 

 condition of men. 



If the surface of the earth consisted of one and the same 

 homogeneous fluid mass, or of strata of rock having the 

 same colour, density, smoothness, and power of absorbing heat 

 from the solar rays, and of radiating it in a similar manner 

 through the atmosphere, the isothermal, isotheral, and isochi- 

 menal lines would all be parallel to the equator. In this 

 hypothetical condition of the Earth's surface the power of 

 absorbing and emitting light and heat would everywhere be 

 the same under the same latitudes. The mathematical consi- 

 deration of climate, which does not exclude the supposition 

 of the existence of currents of heat in the interior, or in the 

 external crust of the earth, nor of the propagation of heat by 

 atmospheric currents, proceeds from this mean, and, as it were, 

 primitive condition. Whatever alters the capacity for absorp- 

 tion and radiation, at places lying under the same parallel of 

 latitude, gives rise to inflections in the isothermal lines. The 

 nature of these inflections, the angles at which the isothermal, 

 isotheral, or isochimenal lines intersect the parallels of latitude, 

 their convexity or concavity with respect to the pole of the 

 same hemisphere, are dependent on causes, which, more or 

 less, modify the temperature under different degrees of 

 longitude. 



The progress of climatology has been remarkably favoured 

 by the extension of European civilization to two opposite 

 coasts, by its transmission from our western shores to a con- 

 tinent which is bounded on the east by the Atlantic Ocean. 

 When, after the ephemeral colonisation from Iceland and 

 Greenland, the British laid the foundation of the first perma- 

 nent settlements on the shores of the United States of Ame- 

 rica, the emigrants (whose numbers were rapidly increased in 

 consequence, either of religious persecution, fanaticism, or 

 love of freedom, and who soon spread over the vast extent of 



