Dr. Forry on the Climate of the United States, Sfc. 19 



the pluviometer, Daniell's hygrometer, the wet bulb, and obser- 

 vations upon the clouds, the clearness of the sky, and the force 

 and direction of the winds.* 



These data were allowed to accumulate in the Medical Bureau 

 for twenty years, before any comprehensive attempt was made 

 to determine their relations to one another, and to deduce from 

 them general laws; and it fell to our lot, under the direction of 

 the present Surgeon General, Thomas Lawson, Esq., to present 

 a systematic arrangement of these isolated facts, embracing the 

 climatology of a vast district, extending from the oldest settle- 

 ments on the Atlantic 'shores to the farthest outposts of civilized 

 occupation, even to the coasts of the Pacific. Thus were pre- 

 sented, under the sanction of the War Department, unlike all 

 other treatises on the same subject, which are generally loosely 

 written and made up of the most vague and general statements, 

 deductions based upon precise instrumental observations. 



As regards the phenomena of superficial terrestrial temperature, 

 let it here suffice to refer to its dependence upon two classes of 

 causes, viz. those resulting from celestial relation, and those pro- 

 duced by geographical position. The former, which may be 

 called the primary constituents of climate, result from the globular 

 figure of the earth, its diurnal motion upon its axis, and the 

 obliquity of its motion in an elliptical orbit in regard to the plane 

 of the equator. Now, if this class of causes solely controlled the 

 phenomena of terrestrial temperature, climates might be classified 

 with mathematical precision ; but the effects produced by solar 

 heat are so much modified by local causes, that the climatic fea- 

 tures of any region can be determined only by observation. It 

 is these last, the secondary constituents of climate, that we have 

 chiefly to do with, in the present inquiry ; and among these geo- 

 graphical or local causes, the following may be regarded as the 

 principal : — 1. The action of the sun upon the surface of the 

 earth. 2. The vicinity of great seas and their relative position. 

 3. The elevation of the place above the level of the sea. 4. The 



* The Medical Bureau procured a number of Daniell's hygrometers from Lon- 

 don — an instrument characterized by beauty, simplicity, and portableness ; but, 

 however well it may be adapted to the humid climate of England, experience 

 soon demonstrated its inapplicability to the arid atmosphere of the United States, 

 with the exception perhaps of our southern borders; and hence has been imposed 

 upon the Department, the necessity of using the wet bulb thermometer in deter- 

 mining the hygrometric condition of the air. 



