Di' Forry on the Climate of the United States, 71 



the level of the sea. 4. The prevalent winds. 5. The form 

 of lands, then' mass, their prolongation toward the poles, 

 their temperature and reflection in summer, and the quantity 

 of snow which covers them in winter. 6. The position of 

 mountains relatively to the cardinal points, whether favour- 

 ing the play of descending currents, or affording shelter 

 against particular winds. 7. The colour, chemical nature, 

 and radiating power of soil, and the evaporation from its 

 surface. 8. The degree of cultivation, and the density of 

 population. 9. Fields of ice, which form, as it were, circum- 

 polar continents, or drift into low latitudes. It is these 

 causes that determine the deviations of the isothermal, iso- 

 eJieimal, and isotheralYmQ^ from the same parallels of latitude. 



In the investigation of the laws of climate, a range of sub- 

 jects so multifarious as to comprise almost every branch of 

 natural philosophy, is embraced; but its ti'ue province is 

 properly restricted to a general view of these subjects, which, 

 if based on legitimate deductions of observed phenomena, 

 should enable us to reduce the infinite variety of appearances 

 presented to us in nature, to a few general principles. It is 

 by means of this generalization that the subject will be ele- 

 vated to the dignity of a science. Climate comprises not only 

 the temperature of the atmosphere, but all those modifica- 

 tions of it which produce a sensible eff"ect on the physical and 

 moral state of man, as well as on all other organic structures, 

 such as its serenity, humidity, changes of electric tension, 

 variations of barometric pressure, its tranquillity as respects 

 both horizontal and vertical currents, and the admixture of 

 terrestrial emanations dissolved in its moisture. Climate, in 

 a word, constitutes the aggregate of all the external physical 

 circumstances appertaining to each locality in its rehation to 

 organic nature. 



In the present inquiry, however, our labours will be re- 

 stricted almost wholly to the mere physical laws of climate. 

 As the climate of every region has an inseparable relation 

 with its physical characters, it follows that, in the investiga- 

 tion of its climatic features, a geographical description be- 

 comes an essential preliminai-y ; but, in the present instance, 

 the country to be described is of so vast an extent as to pre- 



