20 Dr. Forry on the Climate of the United States, ^c. 



prevalent winds. 5, The form of lands, their mass, their prolon- 

 gation toward the poles, their temperature and reflection in sum- 

 mer, and the quantity of snow which covers them in winter. 6. 

 The position of mountains relatively to the cardinal points, wheth- 

 er favoring the play of descending currents or affording shelter 

 against particular winds. 7. The color, 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, circumpolar continents, or drift 

 into low latitudes. It is these causes that determine the devia- 

 tions of the isothermal, isocheimal, and isotheral lines from the 

 same parallels of latitude. 



In the investigation of the laws of climate, a range of subjects 

 so multifarious as to comprise almost every branch of natural 

 philosophy, is embraced ; but its true province is properly re- 

 stricted 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 gen- 

 eralization that the subject will be elevated to the dignity of a 

 science. Climate comprises not only the temperature of the at- 

 mosphere, but all those modifications of it which produce a sen- 

 sible effect 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 admix- 

 ture 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 relation to 

 organic nature. 



In the present inquiry, however, our labors will be restricted 

 almost wholly to the mere physical laws of climate. As the 

 climate of every region has an inseparable relation with its phys- 

 ical characters, it follows that in the investigation of its climatic 

 features, a geographical description becomes an essential prelimin- 

 ary ; but, in the present instance, the country to be described is 

 of so vast an extent as to preclude any thing beyond the most 

 general outlines. It was well remarked by Malte-Brun, that 

 " the best observations upon climate often lose half their value, 

 from the want of an exact description of the surface of the coun- 



